Define epidemiology. Understand historical milestones Epidemiology pass through before attaining its current level. Importance of epidemiology public health professionals and its aspec.

1. EPIDEMIOLOGY
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2. 1. INTRODUCTION TO EPIDEMIOLOGY
OBJECTIVES
At the end of this session students will be able
to:-
• Define epidemiology.
• Understand historical milestones
Epidemiology pass through before attaining its
current level.
• Importance of epidemiology public health
professionals and its aspects.
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3. Definition of Epidemiology
• The word epidemiology comes from the Greek
words
Epi:- meaning on or upon,
Demos:- meaning people, and
Logos:- meaning the study of.
• In other words, the word epidemiology has its
roots in the study of what befalls a population.
• Many definitions have been proposed, but the
following definition captures the underlying
principles and public health spirit of
epidemiology:
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4. Key messages
• Epidemiology is a fundamental science of
public health.
• Epidemiology has made major contributions
to improving population health.
• Epidemiology is essential to the process of
identifying and mapping emerging diseases.
• There is often a frustrating delay between
acquiring epidemiological evidence and
applying this evidence to health policy.
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5. Definition….
• Epidemiology as defined by Last is “the study of
the distribution and determinants of health-
related states or events in specified populations,
and the application of this study to the
prevention and control of health problems”.
• Epidemiologists are concerned not only with
death, illness and disability, but also with more
positive health states and, most importantly, with
the means to improve health.
• The term “disease” encompasses all unfavorable
health changes, including injuries and mental
health.
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6. Definition….
Key terms in this definition reflect some of the
important principles of epidemiology.
Study
• Epidemiology
– is a scientific discipline with sound methods of scientific
inquiry at its foundation.
– is data-driven and relies on a systematic and unbiased
approach to the collection, analysis, and interpretation
of data.
– tend to rely on careful observation and use of valid
comparison groups to assess whether what was
observed.
– However, epidemiology also draws on methods from
other scientific fields, including biostatistics and
informatics, with biologic, economic, social, and
behavioral sciences. 6
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7. Definition….
Epidemiology is often described as the basic science
of public health, and for good reason.
• First, epidemiology is a quantitative discipline
that relies on a working knowledge of
probability, statistics, and sound research
methods.
• Second, epidemiology is a method of causal
reasoning based on developing and testing
hypotheses grounded in such scientific fields as
biology, behavioral sciences, physics, and
ergonomics to explain health-related behaviors,
states, and events.
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8. Definition….
Distribution
• Epidemiology is concerned with the frequency
and pattern of health events in a population.
– Frequency refers not only to the number of
health events in a population, but also to the
relationship of that number to the size of the
population. The resulting rate allows to compare
disease occurrence across different populations.
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9. Definition….
• Pattern refers to the occurrence of health-related
events by time, place, and person.
– Time patterns may be annual, seasonal, weekly, daily,
hourly, weekday versus weekend……..any that may
influence disease or injury occurrence.
– Place patterns include geographic variation,
urban/rural differences, and location of work sites or
schools.
– Personal characteristics include demographic factors
which may be related to risk of illness, injury, or
disability such as age, sex, marital status, and
socioeconomic status, as well as behaviors and
environmental exposures.
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10. Definition….
Determinants
• Epidemiology is also used to search for determinants,
which are the causes and other factors that influence
the occurrence of disease and other health-related
events.
• Epidemiologists assume that illness does not occur
randomly in a population, but happens only when the
right accumulation of risk factors or determinants
exists in an individual.
• Ideally, the findings provide sufficient evidence to
direct prompt and effective public health control and
prevention measures.
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11. Definition….
Health-related states or events
• Epidemiology was originally focused exclusively on
epidemics of communicable diseases but was
subsequently expanded to address endemic
communicable diseases and non-communicable
infectious diseases.
• Indeed, the term health-related states or events may
be seen as anything that affects the well-being of a
population.
• Nonetheless, many epidemiologists still use the term
“disease” as shorthand for the wide range of health-
related states and events that are studied.
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12. Definition….
Specified populations
• Although epidemiologists and direct health-care
providers (clinicians) are both concerned with
occurrence and control of disease, they differ greatly in
how they view “the patient.”
• The clinician is concerned about the health of an
individual; the epidemiologist is concerned about the
collective health of the people in a community or
population.
• In other words, the clinician’s “patient” is the individual;
the epidemiologist’s “patient” is the community.
• While the clinician usually focuses on treating and
caring for the individual, the epidemiologist focuses on
• identifying the exposure or source that caused the illness;
• the number of other persons who may have been similarly exposed;
• the potential for further spread in the community; and
• interventions to prevent additional cases or recurrences. 12
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13. Definition….
Application
• Epidemiology is not just “the study of” health in a population;
it also involves applying the knowledge gained by the studies
to community-based practice.
• Like the practice of medicine, the practice of epidemiology is
both a science and an art.
• To make the proper diagnosis and prescribe appropriate
treatment for a patient, the clinician combines medical
(scientific) knowledge with experience, clinical judgment, and
understanding of the patient.
• Similarly, the epidemiologist uses the scientific methods of
descriptive and analytic epidemiology as well as experience,
epidemiologic judgment, and understanding of local
conditions in “diagnosing” the health of a community and
proposing appropriate, practical, and acceptable public health
interventions to control and prevent disease in the community
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14. Definition….
Summary
• Epidemiology is the study (scientific,
systematic, data-driven) of the distribution
(frequency, pattern) and determinants
(causes, risk factors) of health-related states
and events (not just diseases) in specified
populations (patient is community, individuals
viewed collectively), and the application of
(since epidemiology is a discipline within
public health) this study to the control of
health problems.
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15. 2. Scope of epidemiology
• A focus of an epidemiological study is the
population defined in geographical or other terms;
for example, a specific group of hospital patients or
factory workers could be the unit of study.
• A common population used in epidemiology is one
selected from a specific area or country at a
specific time. This forms the base for defining
subgroups with respect to sex, age group or
ethnicity.
• The structures of populations vary between
geographical areas and time periods.
Epidemiological analyses must take such variation
into account.
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16. 3. Historical Evolution of
Epidemiology
• Although epidemiology as a discipline has
blossomed since World War II, epidemiologic
thinking has been traced from Hippocrates
through John Graunt, William Farr, John Snow,
and others. The contributions of some of
these early and more recent thinkers are
described below.
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17. Historical Evolution…….
Circa 400 B.C.
• Hippocrates attempted to explain disease
occurrence from a rational rather than a
supernatural viewpoint. In his essay entitled
“On Airs, Waters, and Places,” Hippocrates
suggested that environmental and host factors
such as behaviors might influence the
development of disease.
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18. Historical Evolution…….
1662
• Another early contributor to epidemiology
was John Graunt, a London haberdasher and
councilman who published a landmark
analysis of mortality data in 1662. This
publication was the first to quantify patterns
of birth, death, and disease occurrence, noting
disparities between males and females, high
infant mortality, urban/rural differences, and
seasonal variations.
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19. Historical Evolution…….
1800
• William Farr built upon Graunt’s work by
systematically collecting and analyzing Britain’s
mortality statistics.
• Farr, considered the father of modern vital
statistics and surveillance, developed many of the
basic practices used today in vital statistics and
disease classification.
• He concentrated his efforts on collecting vital
statistics, assembling and evaluating those data,
and reporting to responsible health authorities
and the general public.
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20. Historical Evolution…….
1854
• In the mid-1800s, an anesthesiologist named John Snow
was conducting a series of investigations in London that
warrant his being considered the “father of field
epidemiology.”
• Twenty years before the development of the microscope,
Snow conducted studies of cholera outbreaks both to
discover the cause of disease and to prevent its recurrence.
• Snow conducted one of his now famous studies in 1854
when an epidemic of cholera erupted in the Golden Square
of London.
• He began his investigation by determining where in this
area persons with cholera lived and worked.
• He marked each residence on a map of the area. Today, this
type of map, showing the geographic distribution of cases,
is called a spot map.
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21. Historical Evolution…….
19th and 20th centuries
• In the mid- and late-1800s, epidemiological
methods began to be applied in the investigation
of disease occurrence.
• At that time, most investigators focused on acute
infectious diseases.
• In the 1930s and 1940s, epidemiologists
extended their methods to noninfectious
diseases.
• The period since World War II has seen an
explosion in the development of research
methods and the theoretical underpinnings of
epidemiology.
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22. 4. USES OF EPIDEMIOLOGY
Epidemiology and the information generated by epidemiologic
methods have been used in many ways. Some common uses
are.
Assessing the community’s health
• To assess the health of a population or community, relevant
sources of data must be identified and analyzed by person,
place, and time (descriptive epidemiology).
– What are the actual and potential health problems in the
community?
– Where are they occurring?
– Which populations are at increased risk?
– Which problems have declined over time?
– Which ones are increasing or have the potential to increase?
– How do these patterns relate to the level and distribution of
public health services available?
• More detailed data may need to be collected and analyzed
to determine whether health services are available,
accessible, effective, and efficient. 22
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23. USES OF ……..
Making individual decisions
• Many individuals may not realize that they use
epidemiologic information to make daily
decisions affecting their health.
• When persons decide to quit smoking, climb
the stairs rather than wait for an elevator, eat
a salad rather than a cheeseburger with fries
for lunch, or use a condom, they may be
influenced, consciously or unconsciously, by
epidemiologists’ assessment of risk.
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24. USES OF ……..
Completing the clinical picture
• When investigating a disease outbreak,
epidemiologists rely on health-care providers
and laboratorians to establish the proper
diagnosis of individual patients.
• But epidemiologists also contribute to
physicians’ understanding of the clinical
picture and natural history of disease.
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25. USES OF ……..
Searching for causes
• Much epidemiologic research is devoted to
searching for causal factors that influence
one’s risk of disease, by providing enough
information to support effective action .
Ideally, the goal is to identify a cause so that
appropriate public health action might be
taken.
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26. 5. Core Epidemiologic Functions
• In the mid-1980s, five major tasks of
epidemiology in public health practice were
identified:
1. Public health surveillance,
2. Field investigation,
3. Analytic studies,
4. Evaluation, and
5. Linkages.
A sixth task, policy development, was recently
added.
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27. Core Epidemiologic……..
5.1 Public health surveillance
• Public health surveillance is an ongoing,
systematic collection, analysis, interpretation,
and dissemination of health data.
• Is equivalent to monitoring the pulse of the
community.
• Is sometimes called “information for action,”
which is used to portray the ongoing patterns of
disease occurrence and disease potential so that
investigation, control, and prevention measures
can be applied efficiently and effectively.
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28. Core Epidemiologic……..
• This is accomplished through the systematic
collection and evaluation of morbidity and
mortality reports and other relevant health
information, and the dissemination of these
data and their interpretation to those involved
in disease control and public health decision
making.
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29. Core Epidemiologic……..
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Surveillance Cycle
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30. Core Epidemiologic……..
 These reports generally are submitted by
health-care providers,
infection control practitioners, or
laboratories that are required to notify the health
department of any patient with a reportable disease such as
pertussis,
meningococcal meningitis, or
AIDS.
 Other sources of health-related data that are used
for surveillance include reports from investigations of
individual cases and disease clusters,
public health program data such as
immunization coverage in a community,
disease registries, and
health surveys.
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31. Core Epidemiologic……..
While public health surveillance traditionally
has focused on communicable diseases,
surveillance systems now exist that target
Injuries,
Chronic diseases,
Genetic and birth defects,
Occupational and potentially environmentally-
related diseases, and
Health behaviors.
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32. Core Epidemiologic……..
5.2. Field investigation
• Surveillance provides information for action.
• One of the first actions that results from a
surveillance case report or report of a cluster is
investigation by the public health department.
• The investigation may be as limited as:-
– A phone call to the health-care provider to confirm or
clarify the circumstances of the reported case,
– Or it may involve a field investigation requiring the
coordinated efforts of dozens of people to
characterize the extent of an epidemic and to identify
its cause.
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33. Core Epidemiologic……..
• The objectives of such investigations also vary.
• Investigations often lead to the identification of
additional unreported or unrecognized ill persons
who might otherwise continue to spread
infection to others.
– For example, one of the hallmarks of investigations of
persons with sexually transmitted disease is the
identification of sexual partners or contacts of
patients.
• When interviewed, many of these contacts are found to be
infected without knowing it, and are given treatment they
did not realize they needed. Identification and treatment of
these contacts prevents further spread.
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34. Core Epidemiologic……..
• For some diseases, investigations may identify a
source or vehicle of infection that can be controlled
or eliminated.
– For example, the investigation of a case of Escherichia
coli O157:H7 infection in ground beef and sometimes
fruit juice.
• Occasionally, the objective of an investigation may
simply be to learn more about the
– Natural history,
– Clinical spectrum,
– Descriptive epidemiology, and
– Risk factors of the disease before determining what
disease intervention methods might be appropriate
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35. Core Epidemiologic……..
5.3. Analytic studies
• Surveillance and field investigations are usually
sufficient to identify causes, modes of
transmission, and appropriate control and
prevention measures.
• But sometimes analytic studies employing more
rigorous methods are needed.
• Often the methods are used in combination —
with surveillance and field investigations
providing clues or hypotheses about causes and
modes of transmission, and analytic studies
evaluating the credibility of those hypotheses.
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36. Core Epidemiologic……..
• The hallmark of an analytic epidemiologic
study is the use of a valid comparison group.
Epidemiologists must be skilled in all aspects
of such studies,
• Including design,
• Conduct, analysis,
• Interpretation, and
• Communication of findings.
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37. Core Epidemiologic……..
• Design includes
– Determining the appropriate research strategy
and study design,
– Writing justifications and protocols,
– Calculating sample sizes,
– Deciding on criteria for subject selection (e.g.,
developing case definitions),
– Choosing an appropriate comparison group, and
– Designing questionnaires.
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38. Core Epidemiologic……..
• Conduct involves
– Securing appropriate clearances and approvals,
– Adhering to appropriate ethical principles,
– Abstracting records,
– Tracking down and interviewing subjects,
– Collecting and handling specimens, and
– Managing the data.
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39. Core Epidemiologic……..
• Analysis begins with describing the
characteristics of the subjects.
• It progresses to calculation of rates, creation
of comparative tables (e.g., two-by-two
tables), and computation of measures of
association (e.g., risk ratios or odds ratios),
tests of significance (e.g., chi-square test),
confidence intervals, and the like.
• Many epidemiologic studies require more
advanced analytic techniques such as
stratified analysis, regression, and modeling.
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40. Core Epidemiologic……..
• Finally, interpretation involves putting the
study findings into perspective, identifying the
key take-home messages, and making sound
recommendations.
• Doing so requires that the epidemiologist be
knowledgeable about the subject matter and
the strengths and weaknesses of the study.
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41. Core Epidemiologic……..
5.4. Evaluation
• Evaluation is the process of determining, as
systematically and objectively as possible, the
– Relevance,
– Effectiveness,
– Efficiency, and
– Impact of activities with respect to established
goals.
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42. Core Epidemiologic……..
• Effectiveness refers to the ability of a program to
produce the intended or expected results in the field;
effectiveness differs from efficacy, which is the ability
to produce results under ideal conditions.
• Efficiency refers to the ability of the program to
produce the intended results with a minimum
expenditure of time and resources.
• The evaluation itself may focus on
– Plans (formative evaluation),
– Operations (process evaluation),
– Impact (summative evaluation), or
– Outcomes — or any combination of these.
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43. Core Epidemiologic……..
5.5. Linkages
• Epidemiologists working in public health settings
rarely act in isolation.
• During an investigation an epidemiologist usually
participates as either a member or the leader of a
multidisciplinary team. Which includes:-
– laboratorians,
– sanitarians,
– infection control personnel,
– nurses or other clinical staff, and,
– increasingly, computer information specialists.
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44. Core Epidemiologic……..
5.6. Policy development
• The definition of epidemiology ends with the following
phrase: “...and the application of this study to the
control of health problems.”
– While some academically minded epidemiologists have stated
that epidemiologists should stick to research and not get
involved in policy development or even make
recommendations, public health epidemiologists do not have
this luxury.
• Indeed, epidemiologists who understand a problem and
the population in which it occurs are often in a uniquely
qualified position to recommend appropriate
interventions.
• As a result, epidemiologists working in public health
regularly provide input, testimony, and
recommendations regarding disease control strategies,
reportable disease regulations, and health-care policy.
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45. 6. Natural history of disease
OBJECTIVES
By the end of this lecture students should
understand:
1-The definition of natural history of disease.
2-Phases of natural history of disease.
3- The importance of natural history of disease.
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46. • Natural history of disease refers to the progress of a
disease process in an individual over time, in the absence
of medical intervention.
• This process starts from the moment of exposure of an
individual to a casual agent that is capable of causing
disease.
• In the absence of treatment or prevention (intervention),
this process ends with recovery ,disability, or death
The natural history of disease consist of two phases:-
1- Pre-pathogenesis phase.
2- Pathogenesis phase.
Natural history of disease…….
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47. 1- pre-pathogenesis phase
This is the period before the onset of disease (the agent
has not yet entered the human host), but factors favoring
its interaction with the human host are already existing
in the environment.
The causative Factors of disease are classified as:-
1- AGENT (cause of disease).
2- HOST (man).
3- ENVIRONMENT.
Disease results from interaction between these three
factors.
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48. Epidemiologic Triad
Disease results from
interaction between:
Agent
Host
Environment
Concept of Disease causation
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49. 2- Pathogenesis phase
This phase begins with entry of the disease agent in the
susceptible host (man) and it multiplies there and causes
disease.
THE FINAL TERMINATION OF DISEASE MAY BE:-
1-RECOVERY.
2-DISABILITY.
3-DEATH.
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50. THE NATURAL HISTORY OF A DISEASE
STIMULUS to the
HOST
HOST REACTION RECOVERY
interrelation of
Agent, Host and
Environmental
factors
Latent Period (Pre-
symptomatic)
Symptoms,
Signs(Clinical)
with or without Defects,
Disability
PREPATHOGE
NESIS
PERIOD OF PATHOGENESIS
Health Promotion
Specific Protection Early Diagnosis and Prompt Treatment,
Disability Limitation
Rehabilitation
PRIMARY
PREVENTION
SECONDARY
PREVENTION
TREATMENT TERTIARY PREVENTION
(Leavel‘s of Application of Preventive Medicine)
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51. 51
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52. Importance of studying
Natural history of disease
The understanding of this progression from
disease onset to cure or death is important for
epidemiologists.
Natural history is as important as causal
understanding for the prevention and control
of disease.
The earlier you can become aware of the
disease the more likely you will be able to
intervene and save lives.
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53. 7. Levels of disease prevention
Preventive medicine and its importance for
improvement of public health
“Prevention is better then cure”
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54. 7.1 Concepts of prevention
•The goals of medicine are to
•Promote health,
•Preserve health,
•Restore health when it is impaired, and
•Minimize suffering and distress.
•These goals are embodied in the word "prevention".
•Successful prevention depends upon knowledge of
•Causation,
•Dynamics of transmission,
•Identification of risk factors and risk groups,
•Availability of prophylactic or early detection and treatment measures, as
organization for applying these measures to appropriate persons or groups, and
•Continuous evaluation of and development of procedures applied.
It is not necessary (although desirable) to know everything about the
natural history of a disease to initiate preventive measures. Often times,
removal or elimination of a single known essential cause may be sufficient
to prevent a disease. The objective of preventive medicine is to intercept
or oppose the "cause" and thereby the disease process.
The epidemiological concept permits the inclusion of treatment as one of
the modes of intervention.
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55. 7.2 Levels of prevention
•In modern day, the concept of prevention has become broad-based.
•It has become customary to define prevention in terms of four levels:
•Primordial prevention
•Primary prevention
•Secondary prevention
•Tertiary prevention
7.2.1 Primordial prevention
•Primordial prevention, a new concept, is receiving special attention in the
prevention of chronic diseases.
•This is primary prevention in its purest sense, that is, prevention of the
emergence or development of risk factors in countries in which they have
not yet appeared.
•For example, many adult health problems (e.g., obesity, hypertension)
have their early origins in childhood, because this is the time when
lifestyles are formed (for example, smoking, eating patterns, physical
exercise).
•In primordial prevention, efforts are directed towards discouraging
children from adopting harmful lifestyles. The main intervention in
primordial prevention is through individual and mass education.
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56. 7.2.2 Primary prevention
• Primary prevention is an "action taken prior to the onset of
disease, which removes the possibility that a disease will
ever occur". It signifies intervention in the prepathogenesis
phase of a disease or health problem.
• It may be accomplished by measures designed to promote
general health and well-being, and quality of life of people or
by specific protective measures.
• It is far more than averting the occurrence of a disease and
prolonging life. It includes the concept of "positive health", a
concept that encourages achievement and maintenance of "an
acceptable level of health that will enable every individual to
lead a socially and economically productive life”.
• It concerns an individual's attitude towards life and health and
the initiative he/she takes about positive and responsible
measures for himself, his family and his community.
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57. Primary prevention….
•The concept of primary prevention is now being applied to the
prevention of chronic diseases such as coronary heart disease,
hypertension and cancer based on elimination or modification of
"risk factors" of disease.
•The WHO has recommended the following approaches for the
primary prevention of chronic diseases where the risk factors are
established:
A. Population (mass) strategy
B. High-risk strategy
A. Population (mass) strategy:
Another preventive approach is "population strategy" which is
directed at the whole population irrespective of individual levels.
For example, studies have s hown that even a small reduction in the
average blood pressure or serum cholesterol of a population would
produce a large reduction in the incidence of cardiovascular
disease. The population approach is directed towards socio-
economic, behavioral and lifestyle changes. 57
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58. B. High -risk strategy:
• The high-risk strategy aims to bring preventive care to
individuals at special risk.
• This requires detection of individuals at high risk by the
optimum use of clinical methods.
• Primary prevention is a desirable goal. It is worthwhile to recall
the fact that the industrialized countries succeeded in
eliminating a number of communicable diseases like cholera,
typhoid and dysentery and controlling several others like
plague, leprosy and tuberculosis, not by medical interventions
but mainly by raising the standard of living (primary
prevention). And much of this success came even before
immunization became universal routine.
• The application of primary prevention to the prevention of
chronic disease is a recent development. To have an impact on
the population, all the above three approaches (primordial
prevention, population strategy and high-risk strategy) should
be implemented as they are usually complementary.
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59. Primary prevention
• In summary, primary prevention is a "holistic"
approach. It relies on measures designed to
promote health or to protect against specific
disease "agents" and hazards in the environment.
• It utilizes knowledge of the prepathogenesis phase
of disease, embracing the agent, host and
environment.
• The safety and low cost of primary prevention
justifies its wider application. Primary prevention
has become increasingly identified with "health
education" and the concept of individual and
community responsibility for health.
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60. 7.2.3 Secondary prevention
• Secondary prevention can be defined as "action which
halts the progress of a disease at its incipient stage and
prevents complications".
• The specific interventions are early diagnosis (e.g.,
screening tests, case finding programmers) and
adequate treatment.
• By early diagnosis and adequate treatment, secondary
prevention attempts to arrest the disease process;
restore health by seeking out unrecognized disease and
treating it before irreversible pathological changes have
taken place; and reverse communicability of infectious
diseases.
• It may also protect others in the community from
acquiring the infection and thus provide at once
secondary prevention for the infected individuals and
primary prevention for their potential contacts.
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61. Secondary prevention….
• Secondary prevention is largely the domain of clinical
medicine.
• The health programmers initiated by governments are
usually at the level of secondary prevention.
• The drawback of secondary prevention is that the patient
has already been subject to mental anguish, physical pain;
and the community to loss of productivity.
• These situations are not encountered in primary prevention.
• Secondary prevention is an imperfect tool in the control
of transmission of disease.
• It is often more expensive and less effective than primary
prevention. In the long run, human health, happiness and
useful longevity will be achieved at far less expense with
less suffering through primary prevention than through
secondary prevention.
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62. 7.2.4 Tertiary prevention
• When the disease process has advanced beyond its early stages, it
is still possible to accomplish prevention by what might be called
"tertiary prevention".
• It signifies intervention in the late pathogenesis phase.
• Tertiary prevention can be defined as "all measures available to
reduce or limit impairments and disabilities, minimize suffering
caused by existing departures from good health and to promote
the patient's adjustment to irremediable conditions".
• For example, treatment, even if undertaken late in the natural
history of disease may prevent squeal and limit disability.
• When defect and disability are more or less stabilized,
rehabilitation may play a preventable role. Modern rehabilitation
includes psychosocial and medical components based on team
work from a variety of professions. Tertiary prevention extends
the concept of prevention into fields of rehabilitation.
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63. 7.2.5. Rehabilitation
• Rehabilitation has been defined as "the combined and
coordinated use of medical, social, educational and
vocational measures for training and retraining the
individual to the highest possible level of functional
ability".
• It includes all measures aimed at reducing the impact of
disabling and handicapping conditions and at enabling the
disabled and handicapped to achieve social integration.
• Social integration has been defined as the active
participation of disabled and handicapped people in the
mainstream of community life.
• Rehabilitation medicine has emerged in recent years as a
medical specialty. It involves disciplines such as physical
medicine or physiotherapy, occupational therapy, speech
therapy, audiology, psychology, education, social work,
vocational guidance and placement services.
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64. 7.3. The infectious disease cycle
64
The Infectious Cycle
Virologists divide the infectious
cycle into steps to facilitate
their study, but no such
artificial boundaries
Occur.
Infection is the lodgment and
multiplication of organism
in the tissue of host
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65. Objectives / Rationale
• Infection Control is an increasingly
important aspect of health care for
the client and the health care
professional.
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66. Infections and diseases are classified
as:
Endogenous – it originates within.
Includes: metabolic disorders,
congenital abnormalities, tumors,
infections from microorganism within
the body.
Exogenous – originates outside the body;
includes invading pathogens, radiation,
chemical agents, trauma, electric shock,
temperature extremes.
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67. Classification of infections
1. Primary infection: Initial infection with
organism in host.
2. Re-infection: Subsequent infection by same
organism in a host (after recovery).
3. Super-infection: Infection by same organism
in a host (before recovery).
4. Secondary infection: When in a host whose
resistance is lowered by preexisting
infectious disease, a new organism may set
up in infection.
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68. Classification of infections
5. Focal infection: It is a condition where due to
infection at localized sites like appendix and
tonsil, general effects are produced.
6. Cross infection: When a patient suffering from a
disease and new infection is set up from
another host or external source.
7. Nosocomial infection: Cross infection occurring
in hospital.
8. Subclinical infection: It is one where clinical
effects are not apparent.
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69. • Infectious cycle is called the transmission
cycle of disease, or transmission cycle.
• The transmission cycle has different elements:
– The pathogen(Agent): the organism causing the
infection
– The host: the infected person or animal ‘carrying’
the pathogen
– The exit: the method the pathogen uses to leave
the body of the host
– Transmission: how the pathogen is transferred
from host to susceptible person or animal, which
can include developmental stages in the
environment, in intermediate hosts, or in vectors 69
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70. – The environment: the environment in which
transmission of the pathogen takes place.
– The entry: the method the pathogen uses to enter
the body of the susceptible person or animal
– The susceptible person or animal: the potential
future host who is receptive to the pathogen
• To understand why infections occur in a
particular situation, and to know how to
prevent them, the transmission cycles of these
infections must be understood.
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71. Infection Cycle:
• Infective Agent – pathogens include
bacteria, viruses, funguses, rickettsiae,
protozoa.
• Reservoir – where causative agent can
live. Includes the human body, animals,
environment, and fomites or objects
contaminated with infectious material
that contains the pathogens.
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72. • Portal of exit – way for causative agent
to escape from the reservoir.
• Pathogens can leave the body through
urine, feces, saliva, blood, tears, mucous
discharge, sexual secretions, draining
wounds.
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73. • Means of transmission – how it is
transmitted to another host.
• By: Direct Contact – person to person, physical
contact, or contact with body secretions
containing the pathogen.
• Indirect Contact – from contaminated sources
such as food, air, soil, equipment, etc.
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74. • Indirect contact can include touching
contaminated equipment or surfaces,
breathing in droplets carrying airborne
pathogens, or receiving the bite of an
insect carrying the pathogen.
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75. • Portal of entry – way to enter a new
reservoir or host.
• Breaks in the skin or mucous membrane;
respiratory tract; digestive tract;
genitourinary tract; circulatory system.
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76. • Susceptible host – individual who can
contract the disease, unless:
• Defense mechanisms are intact.
• Immune system is functioning.
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77. Necessary Vs Sufficient Cause
• NECESSARY CAUSE: A causal factor whose
presence is required for the occurrence of the
disease.
• SUFFICIENT CAUSE. A causal factor or
collection of factors whose presence is always
followed by the occurrence of the disease.
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78. Mode of Transmission
• Direct Transmission- immediate transfer of the agent
from a reservoir to a susceptible host by direct contact
or droplet spread.
– Touching
– Kissing
– Sexual intercourse
– Blood transfusion
– Trans-placental
• Indirect Transmission- an agent is carried from
reservoir to a susceptible host by suspended air
particles or by animate or inanimate intermediaries.
– Vehicle-born: food, water, towels, ...
– Vector-borne: insect animals, ...
– Airborne: dust, droplets
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79. Some important definitions
• A susceptible cell has a functional receptor for
a given virus – the cell may or may not be able
to support viral replication
• A resistant cell has no receptor - it may or may
not be competent to support viral replication
• A permissive cell has the capacity to replicate
virus - it may or may not be susceptible
• A susceptible AND permissive cell is the only
cell that can take up a virus particle and
replicate it
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