This document discusses key concepts in epidemiology of infectious diseases. It defines epidemiology and provides definitions for infection, infectious disease, and communicable disease. It describes the epidemiological chain of infection and characteristics of infectious agents. It also discusses the natural history of infectious diseases, including generation time, communicable period, and population dynamics such as epidemics, endemics, and pandemics. Finally, it briefly mentions prevention and control strategies.

1. EPIDEMIOLOGY
OF
INFECTIOUS
DISEASE
HARIMUBARGAYARY
Post-GraduateResident
DepartmentofCommunityMedicine

2. Definition :
The study of the occurrence and distribution
of health-related events, states and processes in
specified populations, including the study of the
determinants influencing such processes, and
the application of this knowledge to control
relevant health problems.
EPIDEMIOLOGY
- EPI : among
- DEMOS : people
- LOGOS : study

3. Infection : Entry and development or multiplication of an infectious agent in the human (or
animal) body, with an implied response (e.g., immunological response) on the part of the
human or animal.
*An infection does not always cause illness.
Infectious disease : It is a part of the spectrum of “infection” which is clinically apparent.
It may be contagious or non-contagious.
Communicable disease : An illness due to a specific infectious agent or its toxic products
that arises through transmission of that agent or its products from an infected person,
animal or reservoir to a susceptible host, either directly or indirectly through an
intermediate plant or animal host, vector, or the inanimate environment.

4. CONTAMINATION INFESTATION
Presence of an infectious agent on
a body surface; also on its clothes,
beddings, toys, surgical
instruments or dressings, or other
inanimate articles or substances
including water, milk and food.
For persons or animals the
lodgement, development and
reproduction of arthropods on the
surface of the body or in the
clothing, e.g., lice, itch mite, etc.

5. AGENT
SOURCE / RESERVOIR
PORTAL
OF EXIT
MODE OF TRANSMISSION
PORTAL
OF
ENTRY
SUSCEPTIBLE HOST
EPIDEMIOLOGICA
L CHAIN
OF
INFECTION

6. AGENT
• A substance, living or non-living, or a force, tangible or
intangible, the excessive presence or lack of which may
initiate or perpetuate a disease process.
• The Agent (of Disease) – refers to the causative organism –
the “biological agent”, in the form of virus, bacteria,
protozoa, etc.

7. CHARACTERISTIC OF AGENTS
 Infectiousness: Relative ease with which the agent is transmitted to the
host.
 Infectivity: Ability of the agent to cause infection, i.e., to enter, survive
and multiply in the host.
*Epidemiological measure of infectivity is Secondary Attack Rate
(SAR).

8. SECONDARY ATTACK RATE
( SAR )
Definition:- The number of exposed person developing the
disease within the range of the incubation period,
following exposure to the primary case.
SAR = x 100
Number of exposed persons developing the
disease within the range of the incubation period
_______________________________________
Total number of exposed / “susceptible” contacts

9. CHARACTERISTIC OF AGENTS
 Pathogenicity: Ability of the agent to manifest disease
out of those who have been infected.
 Virulence: Ability of the agent to produce severe disease.
(contd.)

10. SOURCE & RESERVOIR
SOURCE RESERVOIR
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 (or combination of these) in
which an infectious agent lives and
where it reproduces itself in such
manner that it can be transmitted to a
susceptible host”.

11. Disease Source Reservoir
Hookworm
infection
Contaminated soil
with infected larvae
Man
Tetanus Soil Soil
Typhoid fever Faeces / urine of
patients, or
Contaminated
food, milk or water
Case or carrier
Examples of source and reservoir :

12. tS
>
Types of “Reservoir of Infection”
HUMAN Reservoirs Animals and other forms
of reservoir
CASES
Cases are those
who have a
clinically apparent
disease.
CARRIERS
A carrier is a human being who
harbours an infectious agent
and sheds it, thus becoming a
potential source of infection for
other human beings but does
not exhibit any manifestation of
- Animals
- Birds
- Soil
- Inanimate matter, etc.

13. HOST  A person or other living animal, including birds and
arthropods, that affords subsistence or lodgement
to an infectious agent under natural conditions.
Types of Host :-
Obligate host Only host (e.g., man in measles & typhoid fever)
Primary or
host
Hosts in which the parasite attains maturity or passes
its sexual stage.
Secondary or
Intermediate host
Hosts in which the parasite is in a larval or asexual
state.
Transport host A carrier in which the organism remains alive but
does not undergo development.

14. SUSCEPTIBLE HOST
 The person who is at risk of developing an
infection.
 Elderly
 Infants
 Immunocompromised
 ANYONE!

15. Host attributes that affect the
probability of being exposed to the
infectious agent
Host factors that influence the
occurrence of infection and disease
Age (e.g., helminthic infection in young
children d/t habit of oral exploring)
Status of host immunity
Sex (e.g., homemaker females; less
exposure to zoonotic disease)
Age (extremes)
Economic status (e.g., poverty) Genetic make-up
Occupation (e.g., agricultural workers;
more exposure to zoonoses)
Availability and utilization of health
services
Education
Living condition
Lifestyle and behavioural factors
Use of personal protective equipment

16. MODES OF TRANSMISSION
DIRECT TRANSMISSION INDIRECT TRANSMISSION
Direct contact Vehicle – borne (food, water, blood,
etc.)
Droplet infection Vector – borne
i. Mechanical
ii. Biological
Contact with soil Air – borne
i. Droplet nuclei
ii. Dust
Inoculation into skin or mucosa Fomite – borne
Transplacental or vertical Unclean hands and fingers

17. NOSOCOMIAL infection OPPORTUNISTIC infection
Nosocomial (hospital acquired)
infection is an infection
originating in a patient while in
a hospital or other health care
facility.
Infection with organism(s) that
are normally innocuous (e.g.,
commensals in the human) but
become pathogenic when the
body’s immunological defences
are compromised, as in AIDS.

18. ZOONOSES
 They are infections that are normally transmitted between
vertebrate animals, either directly or indirectly through a vehicle or
insect vector.
• Lower vertebrate animals to Man
Anthropo-
• Man to Lower vertebrate animals
Zooanthroponoses
• Man and Lower vertebrate animals
Amphixenosis

19. NATURAL HISTORY
OF
INFECTIOUS DISEASES

20. GENERATION TIME COMMUNICABLE PERIOD
Time interval between receipt of
infection by a host and maximal
infectivity of that host.
The time during which an
infectious agent may be
transferred directly or indirectly
from an infected person to
another person, from an infected
animal to man, or from an
infected person to an animal,
including arthropods.

21. POPULATION DYNAMICS
OF INFECTIOUS DISEASES
Epidemic Epi = upon; demos = people
Occurrence of disease in a defined population or area with a
frequency that is clearly in excess of the normal expectation.
Endemic En = in; demos = people
Constant presence of a disease or infectious agent within a
given geographic area or population group, without
importation from outside.
Sporadic Sporadic = scattered about
Cases occur irregularly, haphazardly from time to time, and
generally infrequently.
Pandemic An epidemic occurring over a wide area, crossing
international boundaries, and usually affecting a large

22. PREVENTION & CONTROL
STRATEGIES