Epidemiology is the science and practice which describes and explains disease patterns in populations, and puts this knowledge to use to improve health. The central paradigm of epidemiology is that patterns of disease in populations may be analyzed systematically to provide understanding of the causes and control of disease. Epidemiology seeks out the differences and similarities ('compare and contrast') in the disease patterns of populations to gain new knowledge. Valid measurement of the frequency of disease and factors which may influence disease, and are therefore potential explanations for the observed patterns, is crucial to the epidemiological goal. The focus of epidemiology is on the pattern of disease and ill-health in the population. Epidemiology combines elements of clinical, biological, social and ecological sciences. Epidemiology is dependent on clinical practice and sciences to make a diagnosis, the starting point of our work. The goal of epidemiology as a science is to understand the causes of disease variation and use this to improve the health of populations and individuals. The goal of epidemiology as a practice is preventing and controlling disease, guiding health and health care policy and planning, and improving health care in individuals. Epidemiological variables should meet the purposes of epidemiology. Epidemiology is based on theories.

1. EPIDEMIOLOGY
by
ORUIKOR GABRIEL JEREMIAH
2023

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EPIDEMIOLOGY
Table of content
CHAPTER ONE: EPIDEMIOLOGY..........................................................................................................................................2
Introduction .................................................................................................................................................................................2
1.1 Definition...............................................................................................................................................................................2
1.2 Scope of epidemiology ......................................................................................................................................................3
1.3 Uses of epidemiology ........................................................................................................................................................3
1.4 Types of epidemiology ......................................................................................................................................................3
CHAPTER TWO: EPIDEMIOLOGICAL TOOLS.....................................................................................................................4
2.1 Incidence............................................................................................................................................................................4
CHAPTER THREE: DESCRIPTIVE EPIDEMIOLOGY...........................................................................................................6
Overview .....................................................................................................................................................................................6
CHAPTER FIVE: EXPERIMENTAL STUDIES .......................................................................................................................9
CHAPTER SIX: EPIDEMIOLOGY VERSUS COMMUNICABLE DISEASES ....................................................................11
CHAPTER SEVEN: LEVELS OF PREVENTION ..................................................................................................................12
CHAPTER EIGHT; EPIDEMIOLOGY OF COMMUNICABLE DISEASES.........................................................................15
CHAPTER NINE: EPIDEMICS...............................................................................................................................................32
9.1 Propagation of Epidemics................................................................................................................................................33
CHAPTER TEN: EPIDEMIOLOGY OF SELECTED DISEASES..........................................................................................38
Bibliography ..............................................................................................................................................................................38

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EPIDEMIOLOGY
CHAPTER ONE: EPIDEMIOLOGY
Introduction
Epidemiology is the science and practice which describes and explains disease patterns in populations,
and puts this knowledge to use to improve health. The central paradigm of epidemiology is that patterns
of disease in populations may be analyzed systematically to provide understanding of the causes and
control of disease. Epidemiology seeks out the differences and similarities ('compare and contrast') in
the disease patterns of populations to gain new knowledge. Valid measurement of the frequency of
disease and factors which may influence disease, and are therefore potential explanations for the
observed patterns, is crucial to the epidemiological goal. The focus of epidemiology is on the pattern of
disease and ill-health in the population. Epidemiology combines elements of clinical, biological, social
and ecological sciences. Epidemiology is dependent on clinical practice and sciences to make a
diagnosis, the starting point of our work. The goal of epidemiology as a science is to understand the
causes of disease variation and use this to improve the health of populations and individuals. The goal of
epidemiology as a practice is preventing and controlling disease, guiding health and health care policy
and planning, and improving health care in individuals. Epidemiological variables should meet the
purposes of epidemiology. Epidemiology is based on theories.
1.1 Definition
Epidemiology may be defined as the study of the distribution and determinants of Health-related states
or events (including disease) in human populations, and the Application of this study to the control of
diseases and other health problems.
• Study; Epidemiology is the basic science of public health. It's a highly quantitative discipline
based on principles of statistics and research methodologies.
• Distribution; Epidemiologists study the distribution of frequencies and patterns of health events
within groups in a population. To do this, they use descriptive epidemiology, which characterizes
health events in terms of time, place, and person.
• Determinants; Epidemiologists also attempt to search for causes or factors that are associated
with increased risk or probability of disease. This type of epidemiology, where we move from
questions of "who," "what," "where," and "when" and start trying to answer "how" and "why," is
referred to as analytical epidemiology.
• Health-related states; although infectious diseases were clearly the focus of much of the early
epidemiological work, this is no longer true. Epidemiology as it is practiced today is applied to
the whole spectrum of health-related events, which includes chronic disease, environmental
problems, behavioural problems, and injuries in addition to infectious disease.
• Populations; One of the most important distinguishing characteristics of epidemiology is that it
deals with groups of people rather than with individual patients.
• Control; Finally, although epidemiology can be used simply as an analytical tool for studying
diseases and their determinants, it serves a more active role. Epidemiological data steers public
health decision making and aids in developing and evaluating interventions to control and
prevent health problems. This is the primary function of applied, or field, epidemiology.
The origin of the word epidemiology is unknown but it is derived from the Greek words meaning study
upon populations. (epi = upon, demos = people, ology = study)

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EPIDEMIOLOGY
Epidemiopathology (pathos is the Greek word for suffering and disease) would be more accurate but
clumsy. Epidemic was used by Hippocrates. Last’s dictionary gives a detailed definition of
epidemiology that includes these words “The study of the distribution and determinants of health-related
states or events in specified populations, and the application of this study to control of health
problems”.
1.2 Scope of epidemiology
Epidemiology is particularly relevant to medicine rather than laboratory science, but the increasing
collaboration between geneticists and epidemiologists is changing the balance
Epidemiology is concerned with disease in populations. Humans live in societies, where behaviour and
attitudes are shaped by interaction among people, which in turn are governed by the conventions and
laws. Epidemiology is, therefore not only a bio-science but also a social science.
Populations exist in a physical environment which is a dominant force in determining health. The study
of life in relation to the environment is ecology, so epidemiology is, in addition, the science of the
ecology of disease.
The science of epidemiology, therefore, combines elements of biology, social sciences and ecology - a
bio-social-environmental science focusing on disease in populations.
1.3 Uses of epidemiology
Currently epidemiology is seen as useful in:
(a) Yielding understanding of what causes or sustains disease in populations
(b) Preventing and controlling disease in populations
(c) Guiding health and health care policy and planning
(d) Assisting in the management and care of health and disease in individuals
(e) Describing the spectrum of the disease
(f) Describing the natural history of the disease
(g) Identifying factors that increase or decrease the risk of acquiring disease
(h) Predicting disease trends
(i) Elucidating mechanisms of disease transmission
(j) Testing the efficacy of intervention strategies
(k) Evaluating intervention programs
(l) Identifying the health needs of a community
(m) Evaluating public health programs
1.4 Types of epidemiology
Basically epidemiology is divided into two; observational and experimental studies.
1. Observational studies
a) Descriptive studies;
• Case report
• Ecological or co relational
b) Analytical studies;
• cross sectional or prevalence
• Case control or case reference
• Cohort or follow up

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EPIDEMIOLOGY
2. Experimental studies
• Randomized controlled trials
• Cluster randomized controlled trials
• Field trials
• Community trials
CHAPTER TWO: EPIDEMIOLOGICAL TOOLS
2.1. Rates
For the public health practitioner interested in determining who is at risk and
monitoring the success of prevention efforts, the most useful measure is a rate. Rates measure the
relative frequency of cases in a population during a specified period of time. The general formula is the
same as for proportions, namely (x/y) k, although here x, y, and k take on different meanings. Rates may
measure incidence (new cases) or prevalence (newly occurring plus pre-existing cases) within a
specified period.
• An incidence rate is the occurrence of new cases of a disease within a defined population at risk
during a specified period of time. In this situation: x is the number of new cases in the defined
population which had its onset during a specified period of time y is the average size of the
defined population at risk in which the disease could occur during the time period specified,
usually at the middle of the time period. k, a constant, depends on convention or is the value such
that the smallest rate in the data set has at least one digit to the left of the decimal point.
• An attack rate is a variant of an incidence rate. In practice, the attack rate will only differ from
the incidence rate if there is a large proportion of persons in the population who are not at risk
(for instance, children who have been successfully vaccinated against measles may be considered
not to be at risk for the disease).
• In a prevalence rate, x is the number of existing cases, new and old, in a defined population
during a specified period (period prevalence) or at a given point in time (point prevalence).
Rates;
• Morbidity e.g., incidence and prevalence rate
• Mortality e.g. crude death rate, infant mortality rate, neonatal mortality rate, perinatal mortality
rate, maternal mortality rate, abortion rate.
Rates- Crude and Specific
2.2 Incidence
The incidence of a disease is the number of new onsets in a population within a specified period of time.
It will be noted that this measure does not make any reference to the size of population studied and
therefore to compare the incidence between a large city and a small village does not make sense.

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EPIDEMIOLOGY
In order to overcome this problem the incidence rate is usually calculated. This divides the incidence by
the size of the population to which it applies.
Thereafter this is usually multiplied by a constant (e.g. 1000, 10000, 100000,
etc.) for ease of interpretation.
Example 1:There were 570 new cases of salmonella food poisoning during 1999 in a city with a
population of 8 million. Incidence = 570 cases per year Incidence rate = 570 / 8 × 106 = No of events in
time period / No in the population = 0.000713/year
Incidence rate / 100,000 / year = 0.000713 × 100,000 = 7.13. Therefore the estimated annual incidence
rate (based on 1999 data) is 7.13 per 100000 persons.
In the example above the denominator population is the average population during the year of interest
and represents an approximation to the number of persons and the time for which they were ‘at risk’ of
developing salmonella food-poisoning. Individuals, for example, who move out of the city or die will no
longer be at risk, while those who move into the city will be newly at risk. It is therefore more accurate
and may practically be possible to determine the time at risk for each individual subject and combine
them to form at total measure of person–time, most commonly person–years at risk. This is obtained
from adding up all the person–time data as the denominator. (At its simplest, if two individuals are
observed, one for three years and the other for five years, then they have contributed in total eight
person–years of observation.

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EPIDEMIOLOGY
CHAPTER THREE: DESCRIPTIVE EPIDEMIOLOGY
Overview
Descriptive studies may be defined as studies that describe the patterns of disease occurrence by time,
place, and person.
Uses of descriptive studies
1. In health planning and administration; descriptive studies and the analysis of their results allow
planners and administrators to allocate resources efficiently.
2. They are also used for hypothesis generation; often providing first important clues about aetiology.
• Case report study
These describe socio-demographic, behavioural and/or medical characteristics for one or more persons
with a similar diagnosis (example: characteristics of children admitted to a hospital with cerebral malaria
during a one-year period). They provide an important link between clinical medicine and epidemiology.
They are often useful for generating hypotheses and examining new diseases. However, conclusions
about aetiology or risk factors cannot be made without having undertaken analytic studies to examine
the expected frequency of exposure to the aetiological or risk factor in a group that does not have the
illness under investigation.
• Ecological study
These may compare disease frequencies among different groups during the same period, or compare
disease frequencies in the same population at different points in time as a function of some exposure.
For instance, the increase over time in the number of persons working as gem miners along the Thai-
Cambodian border (an exposure) parallels the rise in Plasmodium falciparum malaria cases during the
same time period (an outcome).
Ecological studies usually are quick and easy to perform, and can be undertaken with already available
information, but great care is needed to avoid reaching conclusions based on spurious associations.

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EPIDEMIOLOGY
Ecological studies cannot link exposure to outcome in a given individual.
Descriptive studies constitute one of the first steps in outbreak investigation; and should always be
undertaken before initiating further analytic studies.

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EPIDEMIOLOGY
CHAPTER FOUR: ANALYTICAL EPIDEMIOLOGY
Overview
• Analytic studies may be defined as studies used to test hypotheses concerning the relationship
between a suspected risk factor and an outcome, and to measure the magnitude of the association
effect, and its statistical significance. An analytic study always implies a comparison among two
or more groups.
• Cross sectional studies ;these examine the relationship between a disease or other health-related
characteristic and other variables of interest they exist in a population at a given time. The
presence or absence (or the level) of a characteristic is examined in each member of the study
population or in a representative sample. These studies are used to obtain information not
routinely available from surveillance or case series.
• Cross-sectional studies provide no information on the temporal sequence of cause and effect. In
surveys examining the association between an exposure and an outcome, both are measured
simultaneously and it is often hard to determine whether the exposure preceded the outcome or
vice versa. Surveys may simply describe characteristics or behaviours within a study population
(malaria prevalence, vaccine coverage); or may be used to examine potential risk factors (e.g.,
how those who receive vaccination differ from those (who do not).
In general, surveys measure the situation at a given moment – prevalence – rather than the occurrence of
new events – incidence Surveys are not recommended for the study of rare diseases or of diseases with a
short duration, nor are they suited to the study of rare exposures.
• Case control studies
Case-Control Studies proceed conceptually from outcome to exposure; start with groups affected with
the outcome (in the case of a disease, the “ill” group) and groups not affected (“well”) and
retrospectively determine the rates of exposure to a risk factor for each group to see if these rates
differed.
Group of subjects with the disease or other outcome variable (cases), and group of subjects without the
disease or other outcome variable (controls) are identified. Information about previous exposures is
obtained for cases and controls, and the frequency of exposure

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EPIDEMIOLOGY
is compared for the two groups. In case-control studies, both exposure and disease are normally
considered to have occurred prior to enrolment in the study.
In a case-control study, the relationship between exposure and outcome is quantified by calculating the
odds ratio.
• Cohort
A cohort can be defined as a designated group of people who have had a common experience vis-à-vis
exposure, and are then followed up or traced over a period of time.
Cohort studies proceed conceptually from exposure to outcome; starting with exposed and unexposed
groups and following them to see if the rates of occurrence of the outcome in the two groups differ.
Study groups are identified by exposure status prior to ascertainment of their disease status; both
exposed and unexposed groups are then followed prospectively in an identical manner until they develop
the disease under study, until the study ends, or the subjects die or are lost to follow-up. Both cohorts
should have similar characteristics except for the exposure under investigation. Cohort studies differ
from experimental studies in that the investigator does not determine exposure status. This is determined
by genetics or biology (sex, presence or absence of genetic disease, etc.), subject's choice (smoking
behaviour, use of contraceptives, sexual behaviours, food consumption, etc.) or other circumstances
(rural versus urban, socio-economic status etc.).
In some studies, called retrospective cohort studies, exposure and outcome both lie in the past (before
enrolment). The main conceptual element to remember is that the retrospective cohort proceeds from
exposure to disease. In a cohort study, the relationship between exposure and outcome is quantified by
calculating the relative risk for the exposure.
CHAPTER FIVE: EXPERIMENTAL STUDIES
Overview
These are employed to test the efficacy of a preventive or therapeutic measure They consist of trials to
prevent disease (prophylactic trials) or trials to treat established disease processes (therapeutic trials)
They may involve individuals, whole communities or selected groups of individuals Ethical issues must
be considered in the planning and implementation of these studies There is the possibility of
introduction of bias in assessment of outcomes from the expectation of either the investigator or the
participant.
Blinding is a strategy to tackle bias in experimental studies
single blinding
double binding
triple blinding
The last two prevent bias better, but may not always be feasible. The person conducting the study
randomizes the subjects into exposed and unexposed groups and follows them over time to compare
their rates of disease development. Examples may include trials of the efficacy of a new drug compared
with the efficacy of the drug currently in use; or assessment of the efficacy of impregnated mosquito
nets compared with non-impregnated nets. Randomization helps ensure comparability of the groups and
avoids many of the biases inherent in non-experimental studies; for this reason, experimental studies
have been considered as a widely accepted "gold-standard". Experimental studies are nevertheless
expensive; they are not suitable for the study of rare disease outcomes, may take a long time to perform,
often present complex problems of ethics3, or may simply not be feasible (e.g. randomized trials of the
health benefits of breastfeeding). They may also provide results different from those observed under
field conditions.

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EPIDEMIOLOGY
Potential errors in experimental studies
Random error
Random error is the divergence, due to chance alone, of an observation on a sample 4 from the true
population value, leading to lack of precision in the measurement of an association. There are three
major sources of random error: individual/biological variation, sampling error, and measurement error.
Random error can be minimized but can never be completely eliminated since we can study only a
sample of the population; individual variation always occurs and no measurement is perfectly accurate.
Random error can be reduced by careful measurement of exposure and outcome, thus making individual
measurements as precise as possible.
Systematic error (bias)
Bias occurs when there is a tendency to produce results that differ in a systematic manner from the true
values. A study with small systematic bias is said to have high accuracy. Bias (or systematic error) may
lead to over- or underestimation of the strength of an association.5 The sources of bias in epidemiology
are many and over 30 specific types of bias have been identified. The main biases are:
• Selection bias
• Information bias
• Bias due to confounding.
1. Selection bias
Selection bias occurs when there is a systematic difference between the characteristics of the people
selected for a study, or who agree to participate, and the characteristics of those who are not selected, or
who do not agree to participate (e.g. in a study limited to volunteers). Information bias (also called
observation bias)
Information bias occurs when there are quality (accuracy) problems in the collection, recording, coding
or analysis of data among comparison groups. Interviewers might, for example, interview the cases with
more diligence than the interview control, or a person with a disease may recall previous exposures
better than persons who are healthy (this type of bias is called recall bias).
Although selection bias or information bias can usually be corrected at the time of analysis, it is best to
think about possible sources of bias at the time of the study design so that they can be minimized or
avoided.
2. Bias due to confounding
In a study of the association between exposure to a cause (or risk factor or protecting factor) and the
occurrence of the disease, confounding can occur when another factor exists in the study population and
is associated both with the disease and the initial factor being studied. A problem arises if this second
extraneous factor is unequally

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EPIDEMIOLOGY
distributed among the exposure subgroups. Confounding occurs when the effects of two protective or
risk factors have not been separated and it is therefore incorrectly concluded that the effect is due to one
variable rather than the other. For instance, in a study of the association between tobacco smoking and
lung cancer, age would be a confounding factor if the average ages of the non-smoking and smoking
groups in the study population were very different, since lung cancer incidence increases with age.
• Validity and reliability
Validity is an expression of the degree to which a test is capable of measuring what is intended to be
measured. A study is valid if its results correspond to the truth; there should be no systematic error, and
the random error should be as small as possible. Figure 1b illustrates the relationship between the true
value and measured values for low and high validity and reliability (or repeatability). With low
reliability but high validity the measured values are spread out, but the average of the measured values is
close to the true value. High reliability does not ensure validity since the measurements may all be far
from the true value.
CHAPTER SIX: EPIDEMIOLOGY VERSUS COMMUNICABLE
DISEASES
Epidemics
An epidemic is the occurrence in a community of cases of an illness clearly in excess of the normally
expected number of cases.
Types
• Common source
• Point source
• continuous/multiple exposure
• Propagated epidemic
• Investigating an epidemic
These include many of the techniques developed by John Snow. The investigation can be divided into 6
basic steps.
• Verify the diagnosis (clinical/laboratory assessment)
• Verify the existence of an epidemic
• Appraise existing data, Case identification, and characterisation in terms of person, place and
time
• Clinical observation: of cases, to determine types and pattern of symptoms of the dx
• Tabulation and spot maps: Cases may be tabulated by date and time of onset of symptoms,
demographic characterisation etc
• Identification of responsible agent: By estimating the IP, by reviewing the specific symptoms,
laboratory evidence.
• Formulate and test hypothesis
What are the possible sources of infection?

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EPIDEMIOLOGY
What is the likely agent?
What is the most likely method of spread?
Identify type of epidemic
What would be the best approach for control of the outbreak
• Search for additional cases: in other health facilities, investigation of asymptomatic and mild
cases
• Draw conclusions and formulate practical applications e.g. Programs, policies, procedures will
need to be implemented to facilitate long term surveillance and prevention of recurrence.
CHAPTER SEVEN: LEVELS OF PREVENTION
Control is presumed to be the ultimate aim of epidemiology but now prevention seems to be
taking the lead. In a narrow common usage, prevention means the inhibition of the
development of a disease before it occurs. But in the broader sense, prevention also includes
all measures which interrupt or slow the progression of disease and the resulting disability.
Prevention in epidemiology is divided into three main stages.
1. Stage 1: Primary Prevention
Primary prevention has two components: general health promotion (GHP) and specific
measure (SM). GHP includes health education, environmental sanitation and good housing
while SM includes chemoprophylaxis, immunisation and good nutrition. This is the
prevention stage which reduces exposure of an individual or alters the susceptibility of either
being affected by the disease or not.
2. Stage 2: Secondary Prevention
The second stage is the stage of early diagnosis and treatment. This is any measure that will
interfere with the progression of the disease. The measures that constitute early detection
are: screening, case finding, mass x-ray to determine lung diseases and smear to detect
cancer of cervix in a woman.
3. Stage 3: Tertiary Prevention
The third stage of prevention is rehabilitation. This is the alleviation of disabilities from
the disease and attempt to restore effective functioning. Rehabilitation can be divided into
three (3) forms:
• Medical rehabilitation: this is the process of medical care aimed at developing

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EPIDEMIOLOGY
• functional and psychological abilities of the

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individual. Compensatory mechanism is put in place so as to enable the victim to attain self-
dependence and live a full life.
• Social rehabilitation: a part of rehabilitation that aims at the integration of a disabled person
into society by helping him to adjust to the demand of the family, community and occupation
while reducing any economic and social burden that may impede the social rehabilitation
process. It is important in diseases with stigma such as leprosy, pulmonary tuberculosis
and mental illness. Counseling, social evaluation, individual and community counseling,
provision of services including psychiatric services, recreation facilities are components of
social rehabilitation.
• Vocational rehabilitation: this includes provision of those vocational services, vocational
guidance, training, selective placements which are designed to enable a disabled person to
retain a suitable employment. He may require counseling, vocational training, vocational
evaluation, proper placement or being looked after by others.
4. Stage 4: Surveillance
This is the exercise of continuous scrutiny of and watchfulness over the distribution and spread of
infection and the related factors with sufficient accuracy and completeness to provide the basis for
effective control. This idea has three main features namely: systematic collection of all related data
orderly collation and evaluation of each data prompt dissemination of results for action to relevant
authority.
The following are examples of sources of epidemiological data in the surveillance of disease: registration
of deaths, notification of disease and reporting of epidemics, laboratory investigations, data from routine
screening e.g. blood donors, investigation of individual cases and epidemics, epidemiological
surveys, data from clinics, distribution of the animal reservoir and the vector production and distribution
and care of vaccines, serum and drugs, demographic and environmental data and non-medical
statistics.
Objectives of Surveillance
There are five main objectives of surveillance. These are:
• to define the problem
• to define priorities
• to determine strategy

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CHAPTER EIGHT; EPIDEMIOLOGY OF COMMUNICABLE
DISEASES
Definition
A communicable disease is an illness that occurs due to a specific causative agent or its toxic
products which arises through transmission of that agent or its products from a reservoir to
a susceptible host. It could be directly as from an infected person or animal or indirectly
through an intermediate plant or animal host, vector or the inanimate environment.
• Disease-Causing Agents
We shall now consider some of the key points in the definition above:
• Agents and Diseases Caused
Agents Disease caused
Viruses Measles, small pox,
Ricketsiae Typhus organism and tapeworm
Bacteria Spirochetes – Syphilis
Fungi Candidiasis, Tinea Capitis, Histoplasmosis
Protozoa Malaria, Trypanosomiasis, Amoebiasis
Helminthes Nematodes: roundworms, guineaworms, onchocerchiais:
Trematodes and Cestodes: Schizomiasis, Paragomiasis,
Flat worms, Tapeworms, Tinea Sadinasa
Anthropods Gigar
Infectious Agents
An infectious agent is any organism or agent that is capable of producing infection
or infectious diseases. Infection is the successful invasion of the body by micro organisms.
Please note that infection is not the same as infectious disease.
Reservoir
A reservoir is any human being/animal/anthropod/plants/soil or inanimate matter in
which an infectious agent normally lives and multiply and on which it depends
primarily for survival. It reproduces itself in such manner that it can be transmitted to
successive host. Man is the only reservoir of infection from many diseases (man to man).
Occasionally, an animal may serve as the reservoir.
A zoonosis is an infectious disease transmissible under natural

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conditions from vertebrae animal to man. For example, rabies, sleeping

18. P a g e 17 | 34
sickness, yellow fever (jungle type), anthrax, lassa fever (from rat and tape worm).
• Concepts in Communicable Disease
The following concepts are very important in understanding communicable disease.
a) Incubation Period: this is the period between the exposure to an infectious agent and the
appearance of the first signs and symptoms of disease.
1st stage of illness

19. P a g e 18 | 34
Typical period

20. P a g e 19 | 34
Acute illness
(a)
Incubation Period

21. P a g e 20 | 34
(b)
Prodromal Stage (c) (d)

22. P a g e 21 | 34
Exposure
It is worthy of note here that the (d) point is the variable which is the outcome and could be the patient
going into convalescence, chronic illness or death can occur.
Characteristics of organisms that influence diseases/illness formation
There are characteristics that influence disease/illness formation in the body. These are:
• infectivity
• pathogenicity
• virulence
• antigenic power.
Resistance
This is the sum total of body mechanism that provides a barrier to the progress of invasion
or multiplication of infectious agents and damage their toxic products. This is made
possible through immunity.
Carrier
A carrier is someone who though has disease causing organism in his body but does not
show any sign of infection. The carrier has the ability to harbour and disseminate the
parasite without showing any clinical evidence of infection. There are times when even
carriers of a disease are more than those showing the signs of the disease. They often
become chronic carriers but this does not last long. Some of the disease known to have
carriers include: cholera, salmonella typhi, poliomyelitis and diphtheria.
Types of Carrier
1. Incubatory carrier is one that is transferred during incubation period
2. Convalescent carrier is one that is transferred during recovery period
3. Intermittent carrier is one that is on and off
4. Chronic carrier is one in which the individual keeps carrying the disease on for a
long time
5. Healthy carrier is someone who does not show the manifestation at any time but
keeps on transmitting it to people.
Immunity
This is the resistance usually associated with possession of antibodies having specific
actions on the micro-organism concerned with a particular infectious disease or its

23. P a g e 22 | 34
toxin. An individual is considered immune when he possesses specific protective
antibodies or cellular immunity as a result of previous infection or immunisation or
by previous experience. Immunity can be natural or acquired. Natural is inherent in the
individual or specie and it is independent of previous infection. Acquired immunity
can be active and passive. Active acquired immunity can be natural or induced,
while passive acquired may be natural/trans-placental or passive induced.
• Active Immunity: this is the immunity an individual develops as a result of
infection or specific immunisation and usually associated with antibodies or cells
having a specific action on the disease or toxin. This can be acquired through any
of the following.

24. P a g e 23 | 34
After infection e.g. measles
After in-apparent infection e.g. poliomyelitis
After immunisation
• Passive Immunity: this is the transference of antibodies produced in one body to another to
induce protection against disease. This is useful for individual who cannot form antibodies or
for the normal host who takes time to develop antibodies after active immunisation. Here, the
body depends solely on ready-made antibodies. This can be derived from any of the
following:
when an antibody is administered
transfer of maternal antibodies across the placenta
transfer of lymphocytes to induce passive cellular immunity.
• Herd Immunity: this is the level of resistance of a community or group of people to a particular
disease. It provides an immunological barrier to the spread of disease in the human herd.
• Vaccine: this is an immuno-biological substance designed to produce specific protection against
a given disease. It stimulates the production of protective antibody and other immune
mechanisms. It may be prepared from live modified mechanism or inactivated or
killed organisms.
General Methods for the Control of Communicable
Disease
Preventive
measures
Control of
patient,
Epidemic
measures
International
contact
and
measures
environment
(a) Vaccination
against
(a)
Measures
Measures to
limit
(a) Control
of
Epidemic designed to
prevent
spread
of
international
travelers,
spread of
infection
communicable immigrant,
goods,
(b) Chlorination
of
matters to
person
disease which
has
animal products
and
water supplies and to
the
developed
widely in
the means of
transport
© Pasteurisation environment a group
or
of the above
Milk (b)
Keeping
community
within
(b)
Intergovernmental
(d) Control of
rodent
contacts
under
an area, state
or
arrangement,
national
arthropod, animal surveillance
during
nation: laws
(e) Immunisation incubation
periods
(a) Notification
of
©
Monitoring
(f) Public
health
© Keeping
records
occurrence to
the
immunisation
posts
Education under control
until
appropriate
health
especially at
the
(g) Improvement found to be
free of
authority boarders and
ports.
environmental infectious
agents
(b)
Mass
sanitation
and
(d) reporting
to
immunisation
personal; hygiene local authority © Health
education
(h) (e) Isolation (d) Source
and
e.g. malaria,
filariasis,
(f)
Concurrent
contact
investigation
meningococcal disinfection
meningitis,
bacillary
(g) Quarantine
i.e.

25. dysentery. limitation
of
improvement
of
person
exposed to
infection
(h)
Immunisation
of contact
(i)
Investigation
of
(j)
Specific
treatment
(Adapted from “Essentials of Community Health, PHC and Health
Management)
Overview of the Non-communicable Diseases
Non-communicable disease is an illness that occurs due to a specific causative agent or
its toxic products but not transferable from persons to persons. It may be acute or chronic.
Our discussion here will cover the chronic diseases. This will include all ailments or
deviation from normal which have one or the following characteristics:
• permanent disability leaves
• residual disability
• caused by non-reversible
• pathological change
• it requires the special training of the
patient for rehabilitation
• it may be expected to require a long period of supervision, observation and
care.
With the control of communicable disease in some part of the world, a change occurs in the
demographic picture leading to an older population. This is why chronic disease has
become the commonest cause of morbidity and mortality. An estimated 43% of all DALYs
globally were attributable to non-communicable diseases. However in low and middle
income countries the figure was 39%, while in high income countries was 81%. Non-
communicable diseases include cardiovascular, renal, nervous and mental diseases,
musculo-skeletal conditions such as arthritis and allied diseases, chronic non-specific
respiratory diseases (e.g. chronic bronchitis, emphysema, and asthma), permanent results
of accidents, senility, blindness, cancer, diabetes, and obesity and various other metabolic
and degenerative diseases and chronic results of communicable diseases. Disorders of
unknown cause and progressive cause and often labeled “degenerative”.
The Risk Factors of Non-Communicable Disease
There are six (6) major key sets of risk factors that are responsible for major
distribution of non-communicable disease in relation to its morbidity
and premature mortality. These are

27. • use of cigarette and other forms of smoking alcohol abuse
• failure or inability to obtain preventive health services e.g. hypertension control, cancer
detection and management of diabetes
• life-style changes e.g. dietary patterns, physical activity environmental risk factors e.g.
occupational hazards, air and water pollution and possession of destructive weapons
• stress factors.
Problems of Investigating Non-Communicable Disease
There are some problems in the understanding of the natural history of chronic/non-communicable
disease. They include:
• absence of a known agent.
• In some chronic diseases, such as silica in silicosis, the absence of a known agent makes both
diagnosis and prevention difficult. An example of this is cancer
• the multifactoral nature of the aetiology because most chronic diseases are caused by
multiple factors. There is rarely a simple one-to-one cause-effect relationship and in the absence
of a known agent, the term “risk factor/s is used to describe certain factors in relation to a
person’s background or lifestyle.
• Occasionally, it can result from cumulative effects of multiple factors and may be addictive or
synergistic long latent period (incubation period) between the first exposure to suspected cause
and the eventual development of disease which is often difficult to determine.
• It is assumed that what is happening now to someone may result from the effect of past
happenings indefinite onset so that the incidence rate is difficult to calculate.
• Most chronic disease is slow in onset and development and the distinction between diseased
and non-diseased states may be difficult to establish. An example of this is cancer which by
the time the patient seeks medical attention, the damage would have been irreversible or difficult
to treat the differential effect of the factors on the incidence and the cause of the disease.

28. CHAPTER NINE: EPIDEMICS
Definition and Terms in Epidemics
Epidemics can be defined as the occurrence in a community or region or a member of a defined population
or a group having illness of a similar nature in excess of a normal expectancy in that population.
In epidemics, any kind of disease or injury may be involved and there are no universally applicable
number of cases and no clear geographical extent e.g. food poisoning.

29. However, it can affect a large population which cuts across boundaries not really world-
wide (pandemic) and not specific to time.
A disease can be said to be endemic in contrast to epidemics. This is a constant spread of a
disease or an infective agent within a given geographical area. It is the usual prevalence of
a given disease within an area.
Hyperendemic is a term that expresses a persistent intense transmission of the disease e.g.
malaria
Epizootic and Enzootic are expressions that are equivalent of epidemic and endemic as they
apply to animals e.g. epizootic of yellow fever in monkey which precedes that of yellow
fever in man.
Herd Immunity: this is a condition in which community immunity is low. It is the measure
of the proportion of the immune to the susceptible. When the herd is low, the infection can
start and progress because the greater numbers of people are not immuned.
9.1 Propagation of Epidemics
Epidemics only affect a susceptible number of the population. There is an incubation period
before manifestation of symptoms. However, susceptible may develop in apparent
infection. The infectious agent may leave the host during the communicable period which
varies in timing, and duration with each disease. The following are the sequence of
events:
introduction of the agent attack on
susceptible acquisition of immunity
reduction in the number of susceptible decline of the
epidemic.
Introduction attack on acquisition of decline number
of the agent susceptible immunity of susceptible
* The Epidemic Cycle
DECLINE
However, it is worth noting that the cycle can be influenced by any of these factors:
immunity decline, migration and birth/death.

30. Types of Spread
There are two major types of spread in any epidemic. These are common vehicle epidemic and latent
case. We shall consider each of the spread in turns.
The Common Vehicle Epidemic: this is also called the point source. Transmission here may be through
water, food, air or by inoculation. When the epidemic results from a single exposure of the population it
is called a point source epidemic. Sometimes, there may be repeated multiple exposure or a continued
exposure over a period of time e.g. a contaminated well (point source). It is a point source at the closing
up and if from the closing up there is continuous drinking from the source (contaminated well), then it
becomes a multiple source.
Characteristics of common vehicle epidemics
• explosive in onset
• limited in time, place and person (i.e.) there is geographical limitation
• serial transfer or propagation: this usually involves a transfer from host to host. The spread
can be by contact, direct or indirect between the infected and susceptible.
• Route is respiratory, oral-fecal or genital-ingestion
Epidemic
(1)

31. Cases

32. Time
Epidemic Curve
Cases
Time
(Median incubation period)
Typically, a common vehicle epidemic shows a rapid rise and a fall within one
incubation period whereas in propagated epidemic, new cases continue to develop
beyond one incubation period.
Please note the followings:
that a typical point source epidemic may be affected by the development of secondary
source. For example, water that is infected gives diarrhea.
by the continuous contamination of the source, it ceases to give the picture of point source
but multiple sources a disease that has a long incubation period will give the type of a long
epidemic curve with serial transfer on the other hand, a propagated epidemic may look
highly infectious with short incubation period geographical marking can be done to
determine the geographical location of the victims and a spread can occur from centre to
sub- boundary regions. The geographical marking and epidemic curve can help determine
the type of epidemic and the source.
Investigation of Epidemic
Having being exposed to some details about epidemics, it must be stated clearly that there
are due process for investigation and this must be done systematically. It includes the
following:

33. 1. verification of the diagnosis: this includes what type of disease is been viewed, full history,
laboratory investigation, post mortem examination may be required but often a clinical is all
that is needed. People confuse infective hepatitis with yellow fever, hence investigation is
necessary to confirm and rule out
2. confirmation of the existence of an epidemic. This is done by: looking at the previous clinical
records and data, questioning the local people in order to obtain approximate estimate of the
previous incidence of the disease in the area do a mapping to see how scattered/clustering a map
to show that cases are spreading.
3. the identification of the affected persons and their characteristics: who is affected? In terms of
age, sex, name, occupation, etc. Obtain their recent movement, time of onset of their symptoms,
find out whether they were previously immunised,and find out their contacts within the incubation
period of the disease so that you will be able to follow the contact. Here also, you will
endeavour to look for additional cases that may be concealed and not typical in nature. Identify a
common experience shared by all of those affected e.g. do they all go to the same venue when
they ate the food (ceremonies)? Was the water contaminated from source or a sick person visited
the family of recent. All of these provide for epidemiological description of those affected.
4. conduct further laboratory and immunological investigation of the population. Carriers are
deduced e.g. cholera, cerebro-spinal meningitis to identify the type of organism and therapy
sensitivity
5. study the environmental conditions at the time of the outbreak and compare with the previous
condition. Find out if there is any change in water source, weather, food, housing
conditions, population of human beings and any environmental change for epidemic invasion
6. Formulate a hypothesis. Here you will want to find out how it started, its source, method of
transmission, reservoir etc.
7. management of the cases affected. This involves any of the following:
• treatment of cases by health personnel
• rehabilitation facilities: measures are taken to prevent spread and control of epidemics.
• These will include chemotherapies, immunisation, isolation of infected individuals, and
imposition of quarantine so as to reduce movement from one point to the other as well as education
of the community to obtain cooperation.

34. • Permanent control measures are also put in place which includes any of the
following: personal hygiene, health education, water supply, vector control, food
hygiene legislation, continuous vaccination programme.
• Report writing. This is usually from a layman, health planners and a scientific
report.
• Continued surveillance of the population for early detection to reduce the spread.
CHAPTER TEN: EPIDEMIOLOGY OF SELECTED DISEASES
Introduction
THIS CAHPTER IS REFFERED TO THE BOOK “MANAGEMENT OF SELECTED DISESES IN
COMMUNITY HEALTH CARE SETTINGS”
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