This document provides an overview of key concepts in epidemiology. It defines epidemiology and its components of study, distribution, determinants and health-related states or events. It describes epidemiological terminology including infection, contamination, infestation and different types of disease occurrence. It also outlines the epidemiological triad of agent, host and environment factors, modes of disease transmission, measurement tools and levels of disease prevention.

1. Epidemiological Principles
Dr. Navin Adhikari
IM Second Year Resident

2. Content
• Definition
• Terminology
• Case Definition
• Mode of transmission
• Epidemiological Triad
• Natural History of Disease
• Iceberg Phenomenon of Disease
• Level Of Prevention
• Control of Disease
• Measurement in Epidemiology
• Epidemiological Studies
• Investigation Of Outbreaks
• Surveillance

3. Definition
• Epidemiology 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 control
of health problems.
- John M. Last in 1988
Components
• Study - data driven, which relies on a systematic and unbiased
methods to the collection, analysis, and interpretation of
data.
• Distribution - deals with the frequency and pattern of
disease in specified population.

4. • Determinant -any factor, whether event, characteristic, or
other definable entity, that brings about a change in a health
condition or other defined characteristics.
• Health-related states or events - Anythings that affects the
well being of human being. It Includes communicable disease,
non-communicable disease chronic diseases, injuries, birth
defects, maternal-child health, occupational health,
environmental health and human behaviour like amount of
exercise and seat belt use.

5. Distribution of Disease
• It is concerned with the frequency and pattern of health
events in a population.
• Frequency- measurement of frequency of disease, disability
or death. The basic measure of disease frequency is a rate or
ratio.
• Pattern- refers to occurrence of health-related events by
time, place, and person. These are activites of descriptive
epidemiology.
• An important outcome of this study is formulation of
aetiological hypothesis

6. Determinant of Disease
• Determinants are the causes and other factors that influence
the occurrence of disease and other health-related events.
• Uses analytic epidemiological studies to provide the “Why”
and “How” of such events.
• Test aetiological hypotheses and findings provide sufficient
evidence to direct prompt and effective public health control
and prevention measures

7. TERMINOLOGY
INFECTION:The entry and development or multiplication of an
infectious agent in the body of man or animals.
Several levels of infection : colonization (e.g., S. aureus in skin
and normal nasopharynx); subclinical or inapparent infection
(e.g., polio); latent infection (e.g., virus of herpes simplex);
and manifest or clinical infection.
CONTAMINATION: The presence of an infectious agent on a
body surface; also on or in other inanimate articles or
substances including water, milk and food.
INFESTATION: the lodgement, development and reproduction
of arthropods on the surface of the body or in the clothing,
e.g., lice, itch mite.

8. • EPIDEMICS: The unusual occurrence of disease, specific
health-related behaviour or other health related events in a
community or region clearly in excess of "expected
occurrence".
• ENDEMIC :It refers to the constant presence of a disease or
infectious agent within a given geographic area or population
group, without importation from outside; may also refer to
the "usual" or expected frequency of the disease within such
area or population group. e.g. tuberculosis

9. • SPORADIC are those cases that occur irregularly, haphazardly
from time to time, and generally infrequently. e.g., polio,
tetanus. A sporadic disease may be the starting point of an
epidemic when conditions are favourable for its spread.
• PANDEMIC: An epidemic usually affecting a large proportion
of the population, occurring over a wide geographic area
such as a section of a nation, the entire nation, a continent
or the world. eg COVID-19 pandemic.

10. • SOURCE: Is a person, animal, object or substance from which
an infectious agent passes or is disseminated to the host .
Source refers to immediate source of infection & may or may
not be part of reservoir.
• RESERVOIR: Is any person, animal, arthropod,plant, soil or
substance in which an infectious agent lives & multiplies, on
which it primarily depends for survival, & where it
reproduces itself in such a manner that it can be transmitted
to a susceptible host.
HUMAN RESERVOIR: – Cases and Carriers
ANIMAL RESERVOIR: E.g. Rabies, Influenza, Yellow Fever,
Histoplasmosis
RESERVOIR IN NON-LIVING THINGS: E.g. Soil harbour agents
for Tetanus, Anthrax, Coccidiomycosis, Mycetoma

11. Examples

12. Case
• Cases: Persons having particular disease, health disorder or
condition under investigation. It can be clinical cases,
subclinical cases or Latent Infection.
Cases in Epidemiology
• Primary case: First case of communicable disease introduced
into the population unit being studied
• Secondary cases: Cases that develop from contact with the
primary case
• Index case: First case that comes to the notice of the
investigator (first case reported to the health system).

13. Case Definition
• A case definition is clinical criteria usually with confirmatory
laboratory tests, if available, or combinations of symptoms
(subjective complaints), signs (objective physical findings),
and other findings.
• Case definition may have several sets of criteria, depending
on how certain the diagnosis is. For example, during an
investigation of outbreak of COVID-19, a person with a fever
and cough might be classified as having a suspected,
probable, or confirmed case, depending on what evidence is
present.

16. Carriers
• Carriers: Infected person or animal that harbours a specific
agent in the absence of discernible clinical disease, & serves
as a potential source of infection for others
• Carriers are less infectious than cases but are more
dangerous epidemiologically.
• The "Typhoid Mary" is a classic example of a carrier.

17. MODE OF TRANSMISSION
DIRECT TRANSMISSION
1. Direct contact - contact from skin to skin, mucosa to mucosa,
or mucosa to skin of the same or another person. eg: STD
and AIDS, leprosy, leptospirosis, skin and eye infections
2. Droplet infection - direct projection of a spray of droplets of
saliva and nasopharyngeal secretions during coughing,
sneezing e.g. common cold, diphtheria, whooping cough,
tuberculosis, meningococcal meningitis,
3. Contact with soil – eg hookworm, tetanus, mycosis.
4. Inoculation into skin or mucosa e.g rabies, hepatitis B
5. Transplacental (vertical) :TORCH agents (Toxoplasma gondii,
rubella virus, cytomegalovirus and herpes virus), varicella
virus, syphilis, hepatitis B, Coxsackie B and AIDS.

18. INDIRECT TRANSMISSION mechanisms including the traditional
5 F's - "flies, fingers, fomites, food and fluid”
1. Vehicle-borne – through water, food and blood products, ,
acute diarrhoeas, typhoid fever, cholera, polio, hepatitis A,
food poisoning .
2. Vector-borne
a. Mechanical -infectious agent is mechanically transported
by a crawling or flying arthropod through soiling of its feet or
proboscis
b. Biological -infectious agent undergoing replication or
development or both in vector and requires an incubation
period before vector can transmit. three types

19. (i) Propagative : The agent merely multiplies in vector, but no
change in form, e.g., plague bacilli in rat fleas.
(ii) Cyclo-propagative : The agent changes in form and
number, e.g., malaria parasites in mosquito.
(iii) Cyclo-deve/opmental : The disease agent undergoes only
development but no multiplicatione.g.microfilaria in
mosquito
3. Air-borne
a. Droplet nuclei- tuberculosis, influenza, chickenpox
b. Dust - streptococcal and staphylococcal infection,
pneumonia, tuberculosis, Q fever and psittacosis
4. Fomite-borne e.g diphtheria, typhoid fever, bacillary
dysentery, hepatitis A, eye and skin infections
5. Unclean hands and fingers eg staphylococcal and
streptococcal infections, typhoid fever, dysentery, hepatitis A
and intestinal parasites

20. Chain of Infection

21. Incubation Period: It is the time interval between invasion by an
infectious agent and appearance of the first sign or symptom
of the disease in question .
Median incubation period: It is the time required for 50% of
cases to occur.
Generation time: It is the time taken for a person from receipt of
infection to develop maximum infectivity.
Latent period: It is the period from disease initiation to disease
detection, used in noninfectious diseases as equivalent of
incubation period.
Serial interval: It is the gap in onset between primary case (first
case in the community) and secondary case (case developing
through infection from the primary case)

22. Epidemiological Triad
The causative factors of disease may be classified as
1. AGENT,
2. HOST and
3. ENVIRONMENT.
• These three factors are referred to as epidemiological triad.
• The mere presence of agent, host and favourable
environmental factors in the prepathogenesis period is not
sufficient to start the disease in man.
• Interaction of these three factors is required to initiate the
disease process in man.

23. Agent Factor
• Agent factor is defined as a substance, living or non-living, or
a force, tangible or intangible, the excessive presence or
relative lack of which may initiate or perpetuate a disease
process.
Biological agent -viruses, rickettsiae, fungi, bacteria, protozoa
and metazoa
Nutrient agent- excess or deficiency of carbohydrate, protein,
vitamin and minerals
Physical agent- cold, heat, pressure, radiation
Chemical agent- Endogenous: urea, bilirubin
Exogenous:allergens, metals, fumes, dust
Mechanical agent - Exposure to chronic friction and other
mechanical forces may result in crushing, tearing.

24. Host factor
The host factors may be classified as:
(i) Demographic characteristics such as age, sex, ethnicity;
(ii) Biological characteristics such as genetic factors;
biochemical levels of the blood (e.g., cholesterol);
(iii) Social and economic characteristics such as
socioeconomic status, education, occupation, stress
(iv) Lifestyle factors such as personality traits, living habits,
nutrition.

25. Environmental factor
• The environment is defined as "all that which is external to
the individual human host, living and non-living, and with
which he is in constant interaction“.
• three components - physical, biological and psychosocial

27. INFECTIVITY: this is the ability of an infectious agent to invade
and multiply in a host. Equals to Number infected/ Number
exposed .
PATHOGENICITY: this is the ability to induce clinically apparent
illness. Equals to Number of diseased/ Number infected .
VIRULENCE: this is defined as the proportion of clinical cases
resulting in severe clinical manifestations (including sequelae).
It is Number of serious condition & mortality/ Number
diseased .The case fatality rate is one way of measuring
virulence.

28. NATURAL HISTORY OF DISEASE

29. ICEBERG PHENOMENON OF DISEASE
• Disease in a community may be
compared to an iceberg
• Floating tip: What physician sees in
community (Clinical cases)
• Vast submerged portion: Hidden mass
of disease
• Line of demarcation (water surface):
Is between apparent and inapparent
infections.
• Not shown by rabies, tetanus,
measles and rubella

30. LEVEL OF PREVENTION

31. PRIMODIAL PREVENTION
• It is the prevention of the emergence or development of risk
factors in countries or population groups in which they have
not yet appeared .
• Modes of Intervention:
1. Individual Education
2. Mass Education
• Primordial Level is Best level of prevention for Non-
communicable diseases.

32. Primary Prevention
It is the action taken prior to onset of disease, which removes
the possibility that a disease will ever occur Modes of
Intervention:
1. Health Promotion: e.g. Health Education, Environmental
modifications, Nutritional interventions, Lifestyle and
behavioural changes.
2. Specific Protection: immunization (b) use of specific
nutrients (c) chemoprophylaxis (d) protection against
occupational hazards (e) protection against accidents
Primary level of prevention is applied when ‘risk factors are
present but disease has not yet taken place.

33. Secondary Prevention
• It halts the progress of disease at its incipient stage and
prevents complications.
• Modes of Intervention:
1. Early Diagnosis: Detection of disturbances while
biochemical, functional and morphological changes are still
reversible or prior to occurrence of manifest signs and
symptoms
2. Treatment: Shortens period of communicability, reduces
mortality and prevents occurrence of further cases (secondary
cases) or any long term disability –
• Secondary level of prevention is applied when disease has
possibly set in.

34. Tertiary Prevention
• Is applied when disease has advanced beyond early stages.
• It aims to reduce or limit impairments and disabilities,
minimize suffering caused by existing departures from good
health –
• Modes of Intervention:
1. Disability Limitation: It prevents the transition of disease
from impairment to handicap
2. Rehabilitation: Training and retraining of an individual to
the highest possible level of functional ability; It can be
medical, vocational, social or psychological.

35. EXAMPLES
• A patient with fever and cough >3 weeks comes to DOTS Clinic
for ‘Sputum for AFB’: Early diagnosis mode of intervention,
Secondary Level of Prevention (as disease has possibly set in
and sputum for AFB is used to confirm it as a case of
tuberculosis)
• A person sleeps inside a bednet: Specific Protection mode of
intervention, Primary Level of prevention (risk factors, i.e,
mosquitoes are already present, disease has not yet taken place)
• A child coming to Immunisation clinic for OPV Vaccine: Specific
Protection mode of intervention, Primary Level of prevention
(risk factors, i.e, polio infection already present, disease has not
yet taken place) .
• A Urine strip for sugar detection was employed to screen
diabetics in a community: Early diagnosis mode of intervention,
Secondary Level of prevention (screening is meant for early
diagnosis of a disease)

36. STEPS IN DISEASE CONTROL
• Disease control: Is reducing the transmission of disease agent
to such a low level that it ceases to be a public health
problem. Control program in Nepal: TB, Leprosy, Dengue
• Disease elimination: Is complete interruption of transmission
of disease in a defined geographical area, but the causative
organism may be persisting in environment. Elimination
program in Nepal: Poilo, malaria and Kalazar.
• Disease eradication: Is complete ‘extermination’ of organism–
Is ‘tearing out by roots’ of a disease e.g small pox.

37. Measurement in Epidemology
• Mortality – Crude death rate, Specific death rate, Case fatality
rate, Propotional mortality rate, survival rate
• Morbidity – Incidence and Prevalance, Notification rates,
Admission, re-admission and discharge rates , duration of stay
in hospital.
• Disability – DALY,
• Measurement of medical needs, health care facilities,
utilization of health services, other health-related events and
demographic events.

38. Measurement Tool
Rate: Numerator is a part of denominator and multiplier is 1000
or 10,000 or 100,000 or so on
Ratio: Numerator is not a part of denominator and both
numerator and denominator are unrelated .
Proportion: Numerator is a part of denominator and multiplier is
100.Proportion is always expressed in percentage.

39. EXAMPLES

40. INCIDENCE AND PREVALANCE
• Both are morbidity measurement
• Incidence is defined as the ‘no. of new cases’ occurring in a
defined population during a specified period of time .
• Incidence = No. of new cases of a disease in a year/ Total
population at risk X 1000
• It is Rate.
• Special types of incidence rates:
• Attack rate : Incidence rate used when population is exposed
for a small interval of time, e.g. epidemic
• Secondary Attack Rate (SAR): Is no. of exposed persons
developing the disease within range of incubation period,
following exposure to the primary case.
• Incidence can be determined from: Cohort study

41. • Prevalence: Is total current (Old + New) cases in a given population
over a point or period of time .
• Types of prevalence:
Point Prevalence - point of time
Period Prevalence - period of time
• Prevalance = No. of total (new + old) cases of a disease in a year /
total population X 100
• It is a proportion.
• It can be determined from: Cross Sectional Study
• Relationship between Incidence and Prevalence: Given the
assumption that population is stable AND incidence & duration are
unchanging, Prevalence = Incidence × Mean duration of the
disease
• Example incidence of hepatitis A virus (HAV) infections over a
period of 1 year is always greater than its prevalence at a given
point because the disease has a very short duration. In contrast, the
prevalence of HIV or M. tuberculosis infections is always greater
than its incidence because the infection is chronic, and infected

42. EXAMPLES

43. EPIDEMILOGICAL STUDIES

44. DESCRIPTIVE STUDIES
Steps
1.Defining the population to be studied
2. Defining the disease under study
3. Describing the disease by time, place and person
4. Measurement of disease
5. Comparing with known indices
6. Formulation of an aetiological hypothesis

45. ANALYTICAL STUDIES
Case control studies : often called retrospective studies.
Steps in Case control studies
• Statement of the hypothesis
• Selection of cases and control
• Matching between cases and controls
• Measurement of exposure
• Analysis and interpretation.
Odds ratio : is a measure of estimation of disease risk associated
with exposures in case control studies. Also called cross
product ratio. OR = ad/bc for 2x2 contigency table.

47. Cohort is defined as a group of people who share a
common characteristic or experience within a defined
time period (e.g., age, occupation, exposure to a drug or
vaccine, pregnancy, insured persons, etc). Cohort studies
are prospective studies.
Steps in Cohort studies
• Selection of study subject
• Selection of comparison group
• Obtaining information on exposure
• Follow-up
• Analysis and interpretation.

49. • Relative risk (RR) = Incidence among exposed/ Incidence
among non-exposed
• Attributable risk (AR) = (Incidence among exposed –
Incidence among nonexposed)/ Incidence among exposed ×
100

51. • Bias: Is any ‘systematic error’ in an epidemiological study,
occurring during data collection, compilation, analysis and
interpretation.
• Confounding: Any factor associated with both exposure and
outcome, and has an independent effect in causation of
outcome is a confounder. Example In the study of the role of
alcohol in the aetiology of oesophageal cancer, smoking is a
confounding factor because it is associated with the
consumption of alcohol and it is an independent risk factor
for oesophageal cancer.
• Matching: Process of selecting controls in a such a way that
they are similar to cases.Matching eliminates confounding.

53. Preclinical and clinical trials
• These are experimental studies, research activities that
involve the administration of a treatment or prevention
regimen to humans to evaluate its safety and efficacy.

54. Investigation of Outbreak
• Verification of diagnosis:
Is the first step in investigation of an epidemic.It is not
necessary to examine all cases, take sample Do not wait for
laboratory results for epidemiological investigations
• Confirmation of existence of an epidemic:
Compare with disease frequencies during same period in
previous years
• Defining the population at risk:
Obtaining the map of the area. Calculation of appropriate
denominator of population at risk.
• Rapid search for all cases and their characteristics:
Medical survey
Epidemiological case sheet

55. Searching for more cases :this period is usually taken as twice
the incubation period of the disease since the occurrence of
last case
• Data analysis:
Time: Construction of an epidemic curve
Place: Preparation of a spot map
Person: Analysis by age, sex, occupation and other risk factors
• Formulation of hypothesis
• Testing of hypothesis
• Evaluation of ecological factors
• Further investigation of population at risk
• Writing report

57. ISOLATION AND QUARANTINE

58. SURVEILLANCE
• Surveillance: Is the ongoing systematic collection and
analysis of data and the provision of information which
leads to action being taken to prevent and control a
disease, usually one of an infectious nature.
Types
• Passive Surveillance: Data is itself reported to the health
system - Example: A patient with fever coming on his own
to the PHC
• Active Surveillance: Health system seeks out ‘actively’ the
collection of data, i.e., goes out to community to collect
data
• Sentinel Surveillance: ‘identifying missing cases’ and
‘supplementing notified cases. s done in National AIDS
Control Program wherein STD Clinics.

59. Monitoring Versus Surveillance

60. SURVEILLANCE OF COMMUNICABLE
DISEASE IN NEPAL
A. Early Warning And Reporting System (EWARS)
• hospital based sentinel sentinel surveillance system
• Present in 118 hospitals covering all 75 districts of Nepal
• complement the country’s Health Management Information
System (HMIS) by providing timely reporting.
• weekly reporting of detailed line list of cases and deaths
(including "zero" reports)
• six priority diseases/syndromes—Malaria, Kala-azar, Dengue,
Acute Gastroenteritis (AGE), Cholera and Severe Acute
Respiratory Infection (SARI), and other epidemic potential
diseases/syndromes (like enteric fever, Leptospirosis,
Hydrophobia, Chickungunya etc.).

61. • Focuses on immediate reporting (to be reported as soon as
possible within 24 hours of diagnosis) of one confirmed case
of Cholera, Kala-azar, severe and complicated Malaria and one
suspect/clinical case of Dengue as well as 5 or more cases of
AGE and SARI from the same geographical locality in one
week period.
B. Active Surviellience
Active surveillance of infectious disease is done if an outbreak
occurred or is suspected to keep close track to the number of
cases.
C. Water Quality Monitoring and Surveillance
• Regular monitoring of drinking water quality from various
sources and distribution sites
• Regular surveillance of water borne diseases and coordinate
with different stakeholders for quality surveillance.

62. REFERANCES
• Park Textbook of Preventive and Social
Medicine 23rd edition
• Mandell, Douglas, and Bennett’s Principles
and Practice of INFECTIOUS DISEASES
• Mahajan and Gupta Textbook of Preventive
and Social Medicine, 4th edition
• Principles of Epidemiology in Public Health
Practice by CDC
• National Annual Report 75/76

63. THANK YOU