This document defines key terms related to infectious disease epidemiology. It begins by defining infection, contamination, infestation, and host. It then defines and distinguishes between infectious disease, contagious disease, communicable disease, epidemic, endemic, sporadic, pandemic, exotic, and zoonoses. It also defines carrier, classifications of carriers, and modes of disease transmission including direct, indirect, and vector-borne transmission. Finally, it discusses the chain of infection and concepts of reservoirs, susceptible hosts, incubation period, and immunity.

1. INFECTIOUS DISEASE
EPIDEMIOLOGY
Ms. Namita Batra Guin
Associate. Professor

2. FEW DEFINITIONS
 Infection
 Entry and development or multiplication of an infectious
agent in the body of man or animals. Types: colonization,
subclinical and clinical
 Contamination
 Presence of an infectious agent on a body surface, also on
clothes, beddings, toys or other inanimate articles like-
milk, water, food etc.
 Infestation
 The lodgement, development and reproduction of
arthropods on the surface of the body or in the clothing e.g.
Lice, itch mite.
 Host
 A person or other animal, including birds and arthropods
that affords subsistence or lodgement to an infectious
agent under natural conditions. Types: Obligate (only host),
definitive, intermediate, transport.

3. FEW DEFINITIONS
 Infectious Disease
 Clinically manifest disease of man or animals resulting from an
infection.
 Contagious disease
 Disease that is transmitted through direct contact e.g. scabies,
trachoma, STD and leprosy.
 Communicable disease
 Illness due to specific infectious agent or its toxic products capable of
being directly or indirectly transmitted from man to man, animal to
animal or from environment to man or animal.
 Epidemic
 The unusual occurrence of disease, specific health related behaviour
(e.g. smoking) or other health related events (e.g. accidents) clearly
in excess of expected occurrence in a community. It covers both
usual and modern/ slow epidemics. An arbitrary limit of two standard
errors from endemic frequency is used to define the epidemic for
common diseases.

4. FEW DEFINITIONS
 Endemic
 The constant presence of disease or infectious agent within given
geographic area or population group, without importation from
outside. When the conditions are favourable endemic may burst into
epidemic. E.g. hepatitis A, typhoid fever.
 Sporadic
 The cases occur irregularly, haphazardly from time to time, and
generally infrequently. E.g polio, tetanus
 Pandemic
 An epidemic usually affecting a large proportion of the population,
occurring over the wide geographic area such as a continent or
world. E.g. influenza, cholera
 Exotic
 Diseases which are imported into a country in which they do not
otherwise occur e.g. rabies in U.K.

5. FEW DEFINITIONS
 Zoonoses
 Infection or infectious disease transmissible under natural conditions
from vertebrate animals to man. Types: enzootic and epizootic. E.g.
rabies, plague.
 Epizootic
 An outbreak of disease in an animal population. Warning sign that
humans may get affected.
 Enzootic
 An endemic occurring in animals e.g. anthrax, rabies, tick typhus.
 Nosocomial infection
 Infection originating in a patient while in a hospital or other health
care facility. It denotes a new disorder associated with being in a
hospital. It includes infection acquired in the hospital but appearing
after the discharge and also such infections among the staff of the
facility. E.g. infection of surgical wounds, Hep. B etc.

6. FEW DEFINITIONS
 Opportunistic infection
 Infection that takes the opportunity provided by the defect in
the host defence to infect the host and hence cause the
disease. E.g. herpes simplex, M. tuberculosis.
 Iatrogenic disease
 Adverse consequence of a preventive, diagnostic or
therapeutic regimen or procedure that causes impairment,
handicap, disability or death resulting from a physician’s
professional activity or from professional activity of other
health professionals. E.g. reactions to penicillin, hep. B
following blood transfusion.

7. DYNAMICS OF DISEASE
TRANSMISSION

8. CHAIN OF INFECTION

9. SOURCES/ RESERVOIR
 Source: The person, animal, object or substance from
which an infectious agent passes or is disseminated to
the host.
 Reservoir: 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 susceptible
host. Natural habitat of the organism.
 Note: both terms are not always synonymous
 Hookworm infection – reservoir is man but source
of infection is soil.
 In tetanus reservoir and source are same i.e. soil.

10. SOURCES/ RESERVOIR

11. HUMAN RESERVOIR
 Human may be a Case or Carrier
 CASE: A person in the population identified as having
a particular disease
 Clinical case: Clinical illness may be mild, or moderate
, typical or atypical, severe or fatal depending upon the
gradient of involvement.
 Sub-clinical cases: referred to as inapparent, covert,
missed or abortive cases. Disease agent may multiply
but do not manifest sign and symptoms. They
contaminate the environment like clinical cases. They
play dominant role in maintaining the chain of infection
in the community.
 Latent cases: the host does not shed the infectious
agent which lies dormant within the host without the
symptoms. E.g. herpes simplex

12. HUMAN RESERVOIR
 CARRIERS: An infected person or animal that
harbours a specific infectious agent in the
absence of the clinical disease and serves as a
potential source of infection. They are more
dangerous than the cases as they escape the
recognition.
 Elements of carrier state are:
 Presence of the disease agent in the body
 Absence of recognizable symptoms and signs of the disease
and
 Shedding of the disease agent in the discharges or excretion,
thus acting as a source of infection for others

13. CLASSIFICATION OF CARRIER

14. CLASSIFICATION OF CARRIER
 TYPE
 Incubatory Carriers: they shed infectious agent during the
incubation period of the disease. E.g. polio, influenza, diphtheria.
 Convalescent carriers: those who continue to shed the disease
agent during the period of convalescence (recovery). E.g.
cholera, whooping cough, typhoid fever etc. Infect the immediate
environment. A typhoid pt. may excrete bacilli for 6-8 weeks.
 Healthy carriers: they emerge from sub-clinical cases. They are
the victims of sub- clinical infection who developed carrier state
without suffering from overt disease. E.g. cholera, meningococcal
meningitis.

15. CLASSIFICATION OF CARRIER
 BY DURATION
 Temporary carriers: who shed the infectious agent
for short periods of time. It can include incubatory,
convalescent and healthy carriers.
 Chronic carriers: who shed the infectious agent for
indefinite periods. E.g. hep.B, dysentery, malaria etc.
the longer the carrier state, the greater the risk to the
community.

16. CLASSIFICATION OF CARRIER
 By Portal of exit
 Urinary carriers
 Intestinal carriers
 Respiratory carriers
 Nasal carriers
 Occupational status and portal of exit are important
epidemiological considerations

17. ANIMAL RESERVOIR
 May be carriers or cases.
 Diseases transmissible from vertebrates to human is
called as Zoonoses.
 E.g. Rabies, yellow fever and influenza etc.
 Wild birds are important hosts in transmission cycles
of most mosquito borne encephalitis.

18. RESERVOIR IN NON- LIVING THINGS
 Soil and inanimate matter can also act as
reservoir of infection.
 Soil may harbor agents that cause tetanus,
anthrax etc.

19. MODES OF TRANSMISSION

20. DIRECT TRANSMISSION
 DIRECT CONTACT
 Contact from skin to skin, mucosa to mucosa or
mucosa to skin of the same or other person.
 No intermediate agency.
 Reduces the period of survival for the organism and
also ensures a larger dose.
 E.g. AIDS, STDs, leprosy, eye infections.

21. DIRECT TRANSMISSION
 DROPLET INFECTION
 Direct projection of spray of droplets of saliva and
naso-pharyngeal secretions during coughing,
sneezing, speaking and spitting.
 Particles of 5mm or less can penetrate deeply and
reach the alveoli.
 Distance between source and host at which droplet is
spread is 30-60cm.
 E.g. respiratory infections, eruptive fevers, common
cold, T.B. and whooping cough etc.
 Seen in conditions of close proximity
overcrowding and lack of ventilation.

22. DIRECT TRANSMISSION
 CONTACT WITH SOIL
 By direct exposure of susceptible tissue to
disease agent in soil, compost or decaying
vegetable matter.
 Hookworm larvae, tetanus etc.

23. DIRECT TRANSMISSION
 INOCULATION INTO SKIN/ MUCOSA
 Disease agent is inoculated directly in to skin
or mucosa.
 E.g. Rabies virus by dog bite, Hepatitis B by
contaminated needles and syringes etc.

24. DIRECT TRANSMISSION
 TRANSPLACENTAL/ VERTICAL
 Includes TORCH (Toxoplasma gondii, rubella virus,
cytomegalovirus and herpes virus), Hepatitis B, AIDS
etc.
 Non- living agents – thalidomide causes malformations
of the embryo.

25. INDIRECT TRANSMISSION
 Transmission of infections is through some
modes e.g. infective agents – fomites, flies etc.
 Infectious agent must survive outside human
host in external environment and is virulent till
the new host.
 Depends on environmental factors like temp.,
humidity etc.

26. INDIRECT TRANSMISSION
 VEHICLE – BORNE
 Transmission of agents through water, food,
milk, ice, blood serum tissues, organs etc.
 Diseases include: acute diarrheal diseases,
typhoid fever, polio, hepatitis B etc.

27. INDIRECT TRANSMISSION
 VECTOR – BORNE
 Vector- arthropod or any living carrier that transports
an infectious agent to
the susceptible host.
 TYPES – Mechanical and
Biological
 Mechanical Transmission:
 Infectious agent mechanically
transported by crawling or flying arthropods through
soiling its feet, or by passing the organism through
G.I. tract.
THERE IS NO DEVELOPMENT OR
MULTIPLICATION ON OR WITHIN VECTOR

28. INDIRECT TRANSMISSION
 Biological Transmission
 Infectious agent undergo replication or
development or both and require incubation
period before the vector can transfer.
 Types –
 Propagative: Agent multiplies in vector without
changes in its form e.g. plague bacilli
 Cyclopropagative: Agent changes in form and
number. E.g. malarial parasite
 Cyclo-developmental: Agent undergoes only
development but no multiplication. E.g.
microfilariae in mosquito

29. INDIRECT TRANSMISSION
 Chain of transmission of vector borne
diseases:
Man --- arthropod --- Man (malaria)
Mammal --- arthropod --- Man (Rat- rat flea –
man – plague)

30. INDIRECT TRANSMISSION
 AIRBORNE TRANSMISSION
 Indirect transmission of infection existing in
polluted environment through – droplet nuclei
and dust.
 DROPLET NUCLEI- tiny particles (1-10
micron), formed by evaporation of the coughed
or sneezed droplets into the air.
 May remain for longer periods of time.
 Liable to be easily drawn into alveoli of the
lungs.
 E.g. tuberculosis, chickenpox etc.

31. INDIRECT TRANSMISSION
 AIRBORNE TRANSMISSION
 DUST- larger droplets settle down on the floor,
carpet, furniture etc and become the part of
dust.
 During dusting, sweeping – get released in the
air, and inhaled by the host.
 E.g. tubercle bacilli.

32. INDIRECT TRANSMISSION
 FOMITE BORNE
 Contaminated innanimate articles or
substances other than food and water, capable
of harboring and transmitting the infections to
healthy persons.
 Linen, clothes, door handles, taps etc.
 E.g. cholera, typhoid, Hepatitis A, eye and skin
infections etc.

33. INDIRECT TRANSMISSION
UNCLEAN HANDS AND FINGERS
 Most common media
 Transmission –
direct or indirect
 e.g. Typhoid,
Hepatitis A,

34. SUSCEPTIBLE HOST
 Successful parasitism
 Four stages include:
 Portal of Entry: by which agent enters the host. Portal
of entry includes: respiratory tract, ailementary tract,
skin etc.
 Site of Selection: organism reach the appropriate
tissue, where there are optimum conditions for its
survival.
 Portal of exit: to reach the new host and propagate its
species. If no portal of exit – infection is dead- end
infection. E.g. rabies, tetanus etc.
 Organism must survive in external environment

35. SUSCEPTIBLE HOST
 Incubation period: period between the invasion of
the agent an appearance of the first sign and
symptom of the disease.
 Factors determining the incubation period:
 Generation time of the agent
 Infective dose
 Portal of entry
 Individual susceptibility
I.P. few hrs to 2-3 days- staphylococcal food poisoning,
cholera.
I.P. 10 days to 3 weeks- typhoid, measles, mumps etc.
I.P. from weeks to months to years – Hep. A and B,
leprosy etc.

36. SUSCEPTIBLE HOST
 Serial interval: Gap between the onset of primary
case and secondary case is called as serial interval
 Communicable period: time during which agent
may be transferred from an infected person to
another person, from an infected animal to man or
from infected man to animal, including arthropods.

37. SUSCEPTIBLE HOST
 SPECIFIC HOST DEFENCES
 Comes into play, once microorganisms have
breached local defence mechanisms.
 Produced as a result of previous infections
or immunization.
 Classified as :
 Active immunity – humoral, cellular and
combination
 Passive immunity – Normal human Ig, Specific
Human Ig and animal anti toxins or antisera.

38. SUSCEPTIBLE HOST
 Active immunity
 Immunity that the individual develops as a
result of infection or by specific
immunization and is usually associated with
the presence of the antibodies or cells
having specific action on the micro-
organism concerned with the disease.
 Acquired in 3 ways:
 Following clinical infection e.g. chickenpox
 Following sub-clinical infection. E.g. polio
 Following immunization with the antigen.

39. SUSCEPTIBLE HOST
 ACTIVE IMMUNITY
 Immune response
 Primary response- when antigen is
administered for the first time to the human
who has never been exposed to it. Antibody
formed is IgM type. If the stimulus was
sufficient IgG appears in few days
 It is important for the production of memory
cells.

40. SUSCEPTIBLE HOST
 ACTIVE IMMUNITY
 Immune response
 Secondary response/ Booster: production of
antibody is more rapid. Antibody response is
maintained at higher levels.
 It involves the production of IgM and IgG
antibody. But IgG production is more and
larger (bcz of memory cells)

41. SUSCEPTIBLE HOST
 Humoral immunity
 Comes from B-cells.
 Divided into 5 main classes- IgG, IgM, IgE, IgA,
IgD.
 Antibodies are specific to specific infection.
 The specificity is the principle behind the
vaccination.
 But rhinovirus- has many strains- so no single
vaccine.

42. SUSCEPTIBLE HOST
 Cellular immunity
 Helps in combating most of the infections.
 Here macrophages responsible for the
phagocytic actions.
 Responsible for immunity against many
diseases- T.B, brucellosis.

43. SUSCEPTIBLE HOST
 Passive immunity
 When antibody produced in one’s body is
transferred to another to induce protection
against disease.
 It may be induced by:
 By administration of immunoglobulin or
antiserum.
 By transfer of maternal antibodies across the
placenta.
 By transfer of lymphocytes, to induce passive
cellular immunity
IMMUNITY formed is rapid, temporary and there
is no memory cell production.

44. SUSCEPTIBLE HOST
 Herd immunity
 level of resistance of a community or group
of people to a particular disease
 Concerns with the freedom from infection of
individuals within the herd with the only
objective to prevent the transmission among
the individuals
 Provides the immunological barrier to the
spread of disease in the human herd.
 Ongoing immunization program keeps it at
higher level.

45. SUSCEPTIBLE HOST
 IMMUNIZING AGENTS
 Classified as vaccines, immunoglobulins
and antisera.
 VACCINES –
 Live attenuated vaccines
 Killed vaccines
 Toxoids

46. VACCINES

47. VACCINES

48. PREVENTION OF SPREAD OF
INFETIOUS DISEASES
 Measures include:
 NOTIFICATION
 ISOLATION
 DISINFETION
 QUARANTINE
 TREATMENT
 IMMUNIZATION

49. PREVENTION OF SPREAD OF
INFETIOUS DISEASES
 NOTIFICATION: once infectious diseases detected,
they should be notified to local health authority. It
enables early detection of disease outbreaks, which
helps in immediate action.
 Isolation: separation for the period of
communicability of the infected persons from the
others in such places and under such conditions as
to prevent or limit the direct or indirect transmission.
 Disinfection: killing of infectious agents outside the
body by direct exposure to chemical or physical
agents.

50. PREVENTION OF SPREAD OF
INFECTIOUS DISEASES
 QUARANTINE: limitation of freedom of such well
persons or domestic animals exposed to
communicable disease for a period of time not
longer than the longest unusual incubation period.
 Absolute
 Modified
 Segregation

51. DISEASE PREVENTION AND
CONTROL
 Controlling the reservoir: Early diagnosis,
Notification, Epidemiologic investigation, isolation,
treatment and quarantine
 Interruption of transmission: by breaking chain of
transmission. Includes clean practices, adequate
cooking etc.
 Susceptible host’s immunity