John Snow conducted seminal epidemiological work in 1854 during a cholera outbreak in London. By mapping cases of cholera and determining locations of water sources, he discovered cholera was transmitted through contaminated water from a Broad Street pump. Upon its removal, cholera cases decreased suddenly, demonstrating waterborne transmission. This was a major achievement in epidemiology and earned Snow the title of the father of field epidemiology. Epidemiology involves the scientific study of disease patterns in human populations, including the distribution and determinants of health-related states or events. It aims to describe disease occurrence, identify etiological factors, and provide data to support disease prevention, control, and treatment efforts.

1. General
Epidemiology..
Dr Jatin Chhaya
Asst Prof
Department of Community Medicine
SBKS MI & RC

2. History of epidemiology
John Snow and Cholera
• John Snow was conducting the series of investigations in
London that later earned him the title father of field
epidemiology.
• Snow conducted his classical study in 1854 when an
epidemic of cholera developed in the golden square of
London.
• During the time of microscope development, snow
conducted studies of cholera outbreak both to discover
the causes of diseases and prevent its recurrences long
before the birth of bacteriology

3. History of epidemiology
• John Snow strongly believed that cholera is transmitted
through contaminated water.
• He began his investigation by determining where in this area
person with cholera lived and worked.
• He then used this information to map for distribution of
diseases.
• He marked the location and searches the relationship between
cases and water sources.
• He found that cholera was transmitted though contaminated
water which is supplied from pump in broad street.
• After accepting his suggestion, removal of that hand pump was
done and that is result in all of sudden decreases cases of
Cholera.
• This was the major achievement in epidemiology.

5. Introduction
The term epidemiology is derived from the Greek word
epidemic.
– Epi means-Among, upon,
– Demos means study population or people and
– Logos means scientific study.
• So
– it is the scientific study of the disease pattern in human
population.
– In broad sense, it is the study of effects of multiple factors
on human health.
– It is multidisciplinary subject involving those of the
physician, Biologists, Public Health experts, Health
educators etc.

6. The widely accepted definition of
epidemiology is,
"The study of the distribution and determinants
of health related states or events in specified
population and the application of the study to
control of health problems“
(J.M. Last 1988)

7. Components of epidemiology
Distribution of diseases: -
• Health events occur in pattern in community and this pattern
varies from community to community.
• Also health events or diseases condition affect population at
various age groups, different sexes, different subgroups of
population.
• Distributions of events are based on time, place, and person.
We can analyze whether any increases or decreases occur
for a particular condition. Epidemiology addresses itself to a
study of these variations or patterns, which may suggest or
lead to measure to control or prevent the diseases. An
important outcome of this study is formulation of etiological
hypothesis.

8. Components of epidemiology
Determinants of diseases;-
• Epidemiology helps in identifying the causative agent or
the risk/predisposing factors of diseases .
• This is one of the real uses of epidemiology.
Understanding the factors leading to any programs for the
control of those diseases.

9. Aims of epidemiology
According to the International Epidemiological Association
(IEA) Epidemiology has three main aims.
– To describe and analyze diseases occurrence and
distribution in human populations;
– To identify etiological factors in the pathogenesis of
diseases;
– To provide the data essential to the planning,
implementation and evaluation of services for the
prevention, control and treatment of diseases and to the
setting up of priorities among those services.

10. The ultimate aims of epidemiology can be
concluded in to two followings points
• To eliminate or reduce the health problem or its
consequences and
• To promote the health and wellbeing of society as a
whole.

11. Uses of epidemiology
• Investigation of causation of disease.

12. Uses of epidemiology
• Description of the health status of the populations. It
includes proportion with ill Health, change over time,
change with age etc.
• Evaluation of the interventions.
• Planning health services, Public policy and programs.

13. History of epidemiology
In the 1900s epidemiologists extend their methods to non
infectious diseases and studied effect of behaviors and life style
in human health. There are some important achievements in
epidemiology they are;
– John Snow and cholera epidemic in London in 1848-1854.
– Framingham heart study started in 1950 in Massachusetts,
USA and still continuing to identify the factors leading to the
development of the coronary heart diseases.
– Smoking and lung cancer by Doll and Hill in 1964.
– Polio Salk vaccine field trial in 1954 to study the protective
efficacy of vaccine in a million school children.

14. Twoareasofworkforthephysician,viz.communitymedicineand
hospitalmedicine..
Features Clinical Medicine Community Medicine
Diagnosis Made
by
Made by the doctor (Physician) Made by the epidemiologist
Concern Concerned with individual case Concerned with a defined population
Focus Concerned with only sick people
Concerned with both sick and healthy
people
Method to reach
conclusion
Doctor examines the patient Epidemiologist conducts surveys
Interest in
It is arrived at based on signs and
symptoms
It is arrived at based on natural history
of disease
Diagnosis It involves laboratory investigations
It involves epidemiological
investigations
Treatment Doctor decides the treatment
Epidemiologist decides the plan of
action
Aim Treatment is the main aim
Prevention and promotion is the main
aim.
Follow-up It involves follow-up of case It involves the evaluation of program
Equipment
interest
Doctor is interested in technological
advances
Epidemiologist is interested in
statistical values

15. Concept of disease causation
• Germ theory of diseases
• Epidemiological triads
• Multifactorial causation
• Web of causation: Interrelation of different risk factors

16. Epidemiological triad

17. Epidemiological triad
Agent -Biological, chemical, physical, nutritional, Social
Biological agents
These are living agents of disease, viz, viruses,
rickettsiae, fungi, bacteria, protozoa and metazoa. These
agents exhibit certain "host-related" biological properties
such as: (i) infectivity: this is the ability of an infectious
agent to invade and multiply (produce infection) in a
host; (ii) pathogenicity: this is the ability to induce
clinically apparent illness, and (iii) virulence: this is
defined as the proportion of clinical cases resulting in
severe clinical manifestations (including squeal). The
case fatality rate is one way of measuring virulence

18. Nutrient agents
• These are proteins, fats, carbohydrates, vitamins, minerals and
water. Any excess or deficiency of the intake of nutritive
elements may result in nutritional disorders. Protein energy
malnutrition (PEM), anaemia, goitre, obesity and vitamin
deficiencies are some of the current nutritional problems in
many countries.
Physical agents
• Exposure to excessive heat, cold, humidity, pressure, radiation,
electricity, sound, etc may result in illness.
Chemical agents
(i) Endogenous: Some of the chemicals may be produced in the
body as a result of derangement of function, e.g., urea (ureamia),
serum bilirubin (jaundice), ketones (ketosis), uric acid (gout),
calcium carbonate (kidney stones), etc.
(ii) Exogenous: Agents arising outside of human host, e.g.,
allergens, metals, fumes, dust, gases, insecticides, etc. These may
be acquired by inhalation, ingestion or inoculation.

19. Mechanical agents
• Exposure to chronic friction and other mechanical forces
may result in crushing, tearing, sprains, dislocations and
even death.
Social agents
• It is also necessary to consider social agents of disease.
• These are poverty, smoking, abuse of drugs and alcohol,
unhealthy lifestyles, social isolation, maternal
deprivation, etc.

20. Host factor- In epidemiological terminology, the human host is
referred to as "soil" and the disease agent as "seed". 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); blood groups and enzymes;
cellular constituents of the blood; immunological factors; and
physiological function of different organ systems of the body (e.g.,
blood pressure, forced expiratory ventilation). etc.
(iii) Social and economic characteristics such as socioeconomic
status, education, occupation, stress, marital status, housing, etc.
and
(iv) Lifestyle factors such as personality traits, living habits,
nutrition, physical exercise, use of alcohol, drugs and smoking,
behavioural patterns, etc. The association of a particular disease
with a specific set of host factors frequently provides an insight
into the cause of disease.

21. Environmental Factor- Physical, Biological, Psychosocial
Physical: Non-living things and physical factors (e.g., air, water,
soil, housing, climate, geography, heat, light, noise, debris,
radiation, etc) with which man is in constant interaction.
Biological: Living things which surrounds man, including man
himself. The living things are the viruses and other microbial
agents, insects, rodents, animals and plants. These are constantly
working for their survival, and in this process, some of them act
as disease-producing agents, reservoirs of infection, intermediate
hosts and vectors of disease.
Psychosocial: They include cultural values, customs, habits,
beliefs, attitudes, morals, religion, education, lifestyles,
communit life, health services, social and political organization.

24. Dynamic of diseases Transmission
• Source of Infection or reservoir of infection
↓
• Mode of transmission
↓
• Susceptible Host
The source of infection is defined as "the person, animal,
object or substance from which an infectious agent passes
or is disseminated to the host".
A reservoir is defined as "any person, animal, arthropod,
plant, soil or substance in which an infectious agent lives
and multiplies, on which it depends primarily for survival,
and where it reproduces itself in such manner that it can be
transmitted to a susceptible host"

25. The terms reservoir and source are not always synonymous.
For example, in hookworm infection, the reservoir is man,
but the source of infection is the soil contaminated with
infective larvae. In tetanus; the reservoir and source are the
same, that is soil.

26. Reservoir of infection
• Human reservoir
• Case – Clinical, Sub clinical, Latent
• Carrier- Incubatory, Convalescent, Healthy
• Animal Reservoir – Zoonotic
• Reservoir of non- living Things- Soil
• Source of Infection- Secretion/ Excretion of reservoirs,
Some times Reservoirs

27. • Case: “a person in the population or study group identified as
having the particular disease, health disorder or condition
under investigation”
• Subclinical cases: referred to as inapparent, covert, missed or
abortive cases. The disease agent may multiply in the host but
does not manifest itself by signs and symptoms. The disease
agent is, eliminated and contaminates the environment in the
same way as clinical cases. It can be detected only by
laboratory tests, e.g., recovery of the organism, antibody
response, biochemical and skin sensitivity tests.
• Latent case: In latent infection, the host does not shed the
infectious agent which lies dormant within the host without
symptoms (and often without demonstrable presence in blood,
tissues or bodily secretions of the host). Eg: herpes simplex
Terminologies..

28. Terminology
• Primary case: refers to the first case of a communicable
disease introduced into the population unit being studied.
• Index case refers to the first case to come to the attention
of the investigator; it is not always the primary case.
• Secondary cases are those developing from contact with
primary case.
• A suspect case is an individual (or a group of individuals)
who has all of the signs and symptoms of a disease or
condition, yet has not been diagnosed as having the disease
• Zoonosis: The diseases and infections which are
transmissible to man from vertebrates are called zoonosis.
Eg:rabies, yellow fever and influenza

29. • Carrier: an infected person or animal that harbours a
specific infectious agent in the absence of discernible
clinical disease and serves as a potential source of
infection for others“
• The "Typhoid Mary" is a classic example of a carrier.
• The elements in a carrier state are : (a) the presence in the
body of the disease agent (b) the absence of recognizable
symptoms and signs of disease, and (c) the shedding of
the disease agent in the discharges or excretions, thus
acting as a source of infection for other persons.

30. Mode of Diseases transmission
• Direct –
1. Direct Contact - STD and AIDS, leprosy, leptospirosis, skin
and eye infection
2. Droplet Infection - common cold, diphtheria, whooping
cough, tuberculosis, meningococcal meningitis, etc
3. Contact with infected Soil: tetanus, mycosis etc
4. Inoculation into skin or mucosa : The disease agent may be
inoculated directly into the skin or mucosa e.g., rabies virus
by dog bite, hepatitis B virus through contaminated needles
and syringes etc.,
5. Vertical: Transplacentally. Examples include the so-called
TORCH agents (Toxoplasma gondii, rubella virus,
cytomegalovirus and herpes virus), varicella virus, syphilis,
hepatitis B,

31. Indirect
1. Vehicle borne disease: acute diarrhoeas, typhoid fever, cholera,
polio, hepatitis A, food poisoning
2. Vector Borne disease: malaria, plague,
– Biological
– Mechanical
3. Air Borne Disease: Through Droplet nuclei and Dust
– Droplet Nuclei
– Infective Dust
4. Fomite: include soiled clothes, towels, linen, handkerchiefs,
cups, spoons, pencils, books, toys, drinking glasses, door
handles, taps, lavatory chains, syringes, instruments and
surgical dressings.
5. Fingers: staphylococcal and streptococcal infections, typhoid
fever, dysentery, hepatitis A and intestinal parasites.

32. Diseases prevention and control
1. Prevention of source or reservoir
• Early diagnosis
• Notification
• Epidemiological Investigation
• Isolation
• Treatment
• Quarantine
2. Interruption of the disease transmission
3. Prevention of susceptible host
• Immunization
• Health education

33. Epidemiological Investigation
• Verification of diagnosis
• Confirmation of the existence of an epidemic
• Defining the population at risk
• Rapid search for all cases and their characteristics
• Data analysis
• Formulation of hypothesis
• Testing of hypothesis
• Evaluation of ecological factors
• Further investigation of population at risk
• Writing the report

34. Thank you..