This document defines commonly used terms in infectious disease epidemiology and describes concepts related to disease transmission and prevention/control. It defines terms like infection, communicable disease, and incubation period. It describes the chain of infection and discusses reservoirs, modes of transmission, and susceptible hosts. It outlines the objectives and uses of disease surveillance. It also explains methods for preventing and controlling outbreaks like eliminating reservoirs, breaking transmission routes, and protecting susceptibles through immunization, chemoprophylaxis, and general health measures.

1. CHAPTER 5
EPIDEMIOLOGY OF
INFECTIOUS DISEASES
Qassim A. Farah MPH,HN&D

2. COMMONLY USED TERMS
• Infection: Means successful entry,
development and/or multiplication of the
organisms (pathogenic) in the body of a
living being (human or animal).
• Infectious disease: Disease resulting from
an infection.

3. • Subclinical infection: Infection in a dose not
sufficient to result in a disease. An
individual with a subclinical infection may
or may not transmit the disease to others. It
is also called ‘inapparent infection.’
• Identification of such an individual is not
possible unless laboratory investigations are
done.

4. • Opportunistic infection: An infection
caused by those organisms which take the
opportunity provided by the host due to
breakdown of the immune mechanism,
resulting in a disease. It is common in AIDS.
The opportunistic organisms are
Mycobacterium tuberculosis etc.

5. • Nosocomial infection: An infection acquired
by a patient during the stay in the hospital,
either from the hospital staff, or from other
patients or from hospital procedures. For
example, urinary infection following
catheterization. Hepatitis B following
injection or blood transfusion.

6. • Droplet infection: Infection acquired
through the inhalation of droplets or
aerosols of saliva or sputum containing the
pathogens, expelled during sneezing,
coughing, laughing or talking by someone,
harboring the pathogen.

7. • Endogenous infection: The normal
nonpathogenic flora of the body, when
assume the pathogenic character and result
in a disease.
• Primary infection: Infection occurring in an
individual, who is not having any other
infection.

8. • Secondary infection: Infection occurring in
an individual who is already having an
infection of another nature
• Mixed infection: Infection caused by more
than one type of pathogens.

9. • Contamination: Presence of infectious
organism on the surface of the inanimate
objects like clothes, utensils, furnitures,
instruments, vehicles of transmission (like
water, milk, food, blood, etc.) and makes it
impure.
• Pollution: Presence of offensive matter in the
environment For example, air pollution.

10. • Contagious Disease: A disease transmitted
from person to person, when they are in close
physical (skin to skin) contact, For example,
scabies and STDs.
• Communicable Disease: It is being
communicated or transmitted, directly or
indirectly, from man to man, animal to
animal, man to animal, animal to man, or
from environment (through air, soil, food,
water, etc.) to man or animal.

11. • Non Communicable Disease: It is not being
transmitted from person to person. For
example, malnutrition, cancer, diabetes,
hypertension, cardiovascular diseases,
mental illness , etc.
• Vector: It is an arthopod, which spreads the
disease from person to person. For example,
housefly and mosquito, etc.

12. SURVEILLANCE
• It is a process of collection of reliable
information about the status (or occurrence
and spread) of a specific disease in the given
population and the factors related thereto,
for monitoring and reporting on trends in
specific health problems for prevention and
control of that health problem/ disease.

13. Population under surveillance may be a city or
region or nation, the components of this
process are:
• Collection of data
• Analysis of data
• Interpretation of the data
• Reporting of this information
• Action or intervention
• Feedback.

14. OBJECTIVES OF SURVEILLANCE
• The primary objective of surveillance is :
• To monitor the incidence or prevalence of
specific health problem.
• To document their effect in defined
populations and to characterize affected
people and those at greatest risk.

15. Uses of Surveillance
• To highlight the magnitude of the problem in
terms of morbidity and mortality rates.
• To plan for the program intervention
• To monitor the quality of the services.
• To identify the high risk areas for additional
action.
• To identify the outbreaks early for preventive
measures.
• To estimate the needs for drugs.
• To achieve the goals of elimination or
eradication.
• To document the impact of services.

17. DISEAS TRANSMISSION
• Basically there are three pre-requisites for the
transmission of a disease in the community,
namely the reservoir, the susceptible person
and the different routes of transmission from
the reservoir to the susceptible persons.

18. Chain of Infection

19. RESERVOIR
• It is the one in whom the causative organism
lives, multiplies and depends primarily for
its survival (i.e. the natural habitat of the
organism). Such a reservoir may be human
being, animal or soil.
• Its The place where the microorganism
resides, thrives, and reproduces, i.e., food,
water, toilet seat, elevator buttons, human
feces, respiratory secretions.

20. Route of Exit
• The place where the organism leaves the
reservoir, such as the respiratory tract
(nose, mouth), intestinal tract (rectum),
urinary tract, or blood and other body
fluids.

21. Routes of Transmission
• A communicable disease is transmitted from
the reservoir/source to a susceptible person
either directly or indirectly, depending upon
whether they are in close proximity or not.

22. PORTAL OF ENTRY
• PORTAL OF ENTRY The opening where
an infectious disease enters the host’s
body such as mucus membranes, open
wounds, or tubes inserted in body cavities
like urinary catheters or feeding tubes.

23. Direct Modes of Transmission
• There are five direct modes:
• 1. Direct contact: When the reservoir and the
susceptible person are in close, physical, skin
to skin contact. Example, scabies, STDs and
AIDS, (these are also called as contagious
diseases).

24. • 2. Droplet Infection: When the susceptible
person inhales the infected droplets, coming
out of the mouth or nose from the reservoir
during the act of coughing, sneezing, talking
and laughing. For example, pulmonary
tuberculosis, measles, etc.

25. • 3. Contact With The Contaminated Soil: For
example, all soil borne diseases, e.g. tetanus.
• 4. Contact with the animal: Classical example
is rabies transmitted from a rabid animal by
bite and inoculating the virus into the skin or
mucous membrane.
• 5. Trans placental (vertical) transmission:
Means transmission of the disease from the
mother to the fetus through the placenta.

26. Indirect Modes of Transmission
• There are five indirect modes:
• 1. Vehicle Route: The vehicles which are
capable of transmitting the diseases are
water, food, milk, ice-cream, vegetables,
biological products like blood, tissues or
organs (as in organ transplantation).

27. • 2. Vector route: Vector is an arthropod,
capable of transmitting the disease. For
example, mosquitoes, flies, lice, ticks, etc. The
vectors transmit the diseases in the following
ways:

28. • Biting the pathogens percu-taneously. For
example, mosquitoes, ticks, etc.
• Defecation: Scratching in of the infected
feces, into the abrasions of the skin. For
example, epidemic typhus, trench fever
transmitted through the feces of infected
louse.

29. Air-borne Route of Transmission
• Droplets
• Air Pollution

30. A Susceptible Host
• A susceptible person is the one who is
likely/prone to develop the disease. For a
disease to occur in an individual, there must
be a portal of entry, a site of election and
poor defence mechanism.
• Portal of entry may be respiratory route,
alimentary route, per cutaneous route or
genital route. There may be even more than
one route of entry. For example, AIDS,
Hepatitis B, etc.

31. INCUBATION PERIOD
• Definition: It is defined as a ‘period between
the successful entry of a pathogen and the
appearance of the first clinical sign or
symptom of the disease’, in an individual. It
is also called ‘intrinsic incubation period.

32. Factors influencing the incubation
period: These are virulence
• Factors influencing the incubation period:
These are virulence of the pathogens,
infective dose and the susceptibility of the
individual. In short, the incubation period
depends upon the host-parasite relationship.

33. • Latent period: It is similar to incubation
period but with reference to non
communicable diseases such as diabetes,
hypertension, cancer, etc. It is a period
between the initiation of the disease and the
detection of the disease.

34. • Window period: It is the period between the
entry of the pathogen and the production of
the antibodies. It is 2 to 8 weeks in
HIV/AIDS. ELISA test is negative during this
period. But infected person is infectious to
others during this period.

35. PREVENTION AND CONTROL OF AN
EPIDEMIC DISEASE
• The basic concept of control of an outbreak of
the disease is to break the weakest link in the
chain of transmission of the disease. These
are three major steps:
• 1. Elimination of reservoir
• 2. Breaking the channel of transmission
• 3. Protection of susceptible.

36. ELIMINATION OF RESERVOIR
• Elimination of environmental reservoir, such
as air, water.
• Elimination of animal reservoir means
keeping the animals away from the human
habitation.
• Early diagnosis
• Epidemiologic investigations
• Isolation

37. Direct Modes of Transmission
• Direct modes of transmission can be
broken as follows:
• Contact: Transmission can be broken by
avoiding physical, skin to skin contact with
infectious, (contagious) cases
• Droplet infection
• Soil borne transmission can be prevented by
using shoes while walking.

38. Indirect Modes of Transmission
• Vector route of transmission broken by
control of vectors.
• Air-borne transmission can be prevented by
controlling air pollution, control of infected
dust in the hospital wards and other
measures such as adequate ventilation, etc.

39. PROTECTION OF SUSCEPTIBLE
• Immunization Programs
• General measures such as improvement in
the quality of life such as
• good living
• better nutrition
• clean sanitation.

40. • Immunization refers to administration of
vaccines to increase the resistance of the
susceptible host against specific vaccine-
preventable infections. For example, measles
vaccination helps to protect the child from
measles infection, and BCG vaccination gives
some protection from tuberculosis.

41. • Chemoprophylaxis refers to the drugs given
to exposed and susceptible hosts to prevent
them from developing an infection. For
example, individuals from non-malarial areas
who are going to a malaria endemic area can
take a prophylactic drug to prevent them
from developing the disease if they become
infected with malaria parasites from a
mosquito bite.

42. • Maintaining a healthy lifestyle Proper
nutrition and exercise improves a person’s
health status, supports the effective
functioning of their immune system, and
increases resistance to infection.

43. ANY QUESTION