This document provides an overview of infectious disease epidemiology. It begins with a brief history of some major infectious disease outbreaks and their impacts. It then discusses concepts and definitions relevant to infectious disease epidemiology, including reservoirs, modes of transmission, epidemiological triad, and terminology. The document outlines the importance of studying infectious disease epidemiology and highlights current challenges like antimicrobial resistance and emerging/re-emerging pathogens. It also summarizes successes in disease eradication/elimination and the ongoing global burden of infectious diseases.

1. Infectious Disease Epidemiology
Lt Col A S Kushwaha

2. • Infectious diseases - History
• Why study Infectious diseases
• What is infectious disease epidemiology
• Concepts / definitions – IDE

3. HISTORY
• WORST SCOURGES OF MANKIND
• THREAT TO HUMAN SURVIVAL
• KILLED MORE MEN THAN ALL THE WARS
• CAHNGED THE COURSE OF HISTORY
10/15/2013
3

4. • 14th century
• 1819• 1831
• 1854-56
• 1899-1902 -
- Europe - plague kills 20-45 % of the
world’s population
- 50 million deaths due to H1N1
spanish Flu
- Cairo – 13 % of population
succumbs to cholera
- Crimean war – deaths due to
dysentery were 10 times higher
than deaths due to casualties
Boer War – deaths due to
dysentery were 5 times higher
than deaths due to casualties

6. INFECTIONS
• Pre 19th Century – Era of Microbes (we didn’t
know much about them)
• 20th Century – Man – upper hand Vs Microbes
(antibiotics, vaccines, safe water, sanitation)

7. Because infectious diseases have been largely
controlled in the United States, we can now
close the book on infectious diseases.”
William Stewart, MD U.S. Surgeon General, 1967

8. Microbes are smarter
Microbes and vectors swim in the evolutionary
stream, and they swim faster than we do.
Bacteria reproduce every 30 minutes. For them,
a millennium is compressed into a fortnight.
They are fleet afoot, and the pace of our
research must keep up with them, or they will
overtake us. Microbes were here on earth 2
billion years before humans arrived, learning
every trick for survival, and it is likely that they
will be here 2 billion years after we depart
(Krause 1998).

9. 15 million deaths

10. DALYs INFECTIOUS DISEASES

11. 21st Century
• Microbes are back in news
• Resistance
• Newer pathogens
• Changing environment – warming
• Bioterrorism

12. SUCCESSES
• Eradication of Smallpox in 1977
• Elimination of Poliomyelitis from the Western
Hemisphere in 1994
• Potential elimination of global poliomyelitis in
the next 5 to 10 years
• Potential elimination of measles in the next 10
to 20 years
• Vaccines in development for prevention of
diarrheal diseases, cervical cancer (HPV)

13. Mortality due to infectious diseases

15. CHALLENGES
• More pathogens have been identified than the
drugs developed
• Many pathogens no longer respond to drugs
• Human activity has accelerated this imbalance
• HIV

16. Wonder drugs to Super bugs

18. Global scenario

19. Emerging & Re-emerging infections

21. Global Burden of infectious diseases
• One death in three of the 54 million deaths worldwide is
from an infectious cause
• Virtually all of these deaths are in developing areas of the
world – mainly India and sub-Saharan Africa
• Disproportionately affect children
• Many of the developing world deaths are due to preventable
causes
–
–
–
–
Pneumonia and Diarrhea – account for 40% of these deaths
Tuberculosis
Measles
Malaria

22. • Of the seven biggest killers worldwide, TB, malaria,
hepatitis, and, in particular, HIV/AIDS continue to
surge
– HIV/AIDS and TB likely to account for the
overwhelming majority of deaths from infectious
diseases in developing countries by 2020
– Acute lower respiratory infections, diarrheal
diseases and measles appear to have peaked at
high incidence levels

23. Emerging AIDS Related Infectious Diseases
1. Pneumocystis carinii pneumonia
2. Tuberculosis
3. Mycobacterium-avium complex
4. Kaposi’s sarcoma (HHV-8)
5. HSV-2
6. Cryptosporidium
7. Microsporidium
8. Cryptococcus neoformans
9. Penicillium marneffei
10. Disseminated salmonella
11. Bacillary angiomatosis (Bartonella henselae)
12. HPV

24. Some Emerging Non- AIDS related Infectious Diseases
1. SARS
2. West Nile disease
3. Variant CJD disease
4. Monkey pox
5. Ebola and Marburg
viruses
6. Dengue
7. Influenza H5/N1 (?)
8. Hanta virus
9. E. Coli O157 :H7
10. Antibiotic-resistant
– Pneumococci
– Staph-aureus
– Gonococci
– Salmonella
11. Cryptosporidium
12. Anthrax

25. Millions of deaths, worldwide, all ages,

26. 2000
10/15/2013
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27. 2007
10/15/2013
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28. 10/15/2013
28
Charrel et al. 2007. N Engl J Med 356;8

29. Factors Leading to Emergence of
Infectious Diseases
•
•
•
•
•
•
AIDS
Population growth
Speed and ease of travel
Dam building
Global climate change
Increased antibiotic use
for humans and animals
• Encroachment of human
populations on forest
• Industrial commercial
agriculture
• War and social disruption
• Relocation of animals
• Growth of daycare
• Aging of the population
• Human-animal contact

30. Emerging Infectious Diseases Related
to Animal Contact
Disease
1. HIV-1, LHIV-2
2. Influenza
3. Ebola
4. Marburg
5. Hanta virus
6. Arena virus
7. Variant CJD
8. Cryptosporidia
9. Hendra virus
10. Nipah virus
11. SARS
12. Monkey pox
Animal
Primates (Africa)
Water fowl, pigs, chickens (N5/N1)
Primates (“bush meat”)
Primate (“bush meat”)
Deer mouse
Various rodents
Cattle
Cattle
Fruit bats
Pigs, fruit bats
Civet cats
Prairie dogs

31. Epidemiology
• Study of distribution & determinants of
disease and health related events and
its application in control and
prevention.

32. What is infectious disease
epidemiology?
•
•
•
•
Epidemiology
Deals with one population
Risk  case
Identifies causes
Infectious disease epidemiology
 Two or more populations
 A case is a risk factor
 The cause often known
(www)

33. Importance of Studying Communicable
Diseases Epidemiology
• Changes of the pattern of infectious diseases
• Discovery of new infections
• The possibility that some chronic diseases
have an infective origin.

34. What is infectious disease epidemiology?
Two or more populations
Humans
Infectious agents
Helminths, bacteria, fungi, protozoa, viruses, prions
Vectors
Mosquito (protozoa-malaria), snails (helminths-schistosomiasis)
Blackfly (microfilaria-onchocerciasis) – bacteria?
Animals
Dogs and sheep/goats – Echinococcus
Mice and ticks – Borrelia
(www)

35. What is infectious disease
epidemiology?
A case is a risk factor …
Infection in one person can be transmitted to others
(www)

36. Concepts Specific to Infectious
Disease Epidemiology
Attack rate, immunity, vector,
transmission, carrier, subclinical
disease, serial interval, index case,
source, exposure, reservoir,
incubation period, colonization,
generations, susceptible, nonspecific immunity, clone, resistance,
repeat episodes …

38. Infectious Disease Epidemiology

39. Objectives
• The epidemiologic triad
• Definition of communicable diseases
• Importance of studying communicable diseases
epidemiology
• Terminology
• Dynamics of disease transmission (chain of
infection):
– Human reservoir or source
– Modes of transmission
– Susceptible host

40. Epidemiologic triad
•Demographic characteristics
•Biological characteristics
•Socioeconomic characteristics
Host
Agent
•Biological agents
•Physical agents
•Chemical agents
•Nutrient agents
•Mechanical agents
•Social agents
Environment
•Physical environment
•Biological environment
•Social environment

41. Infectious Disease Model
Pathogen
Host
Disease
Environment

42. Definition of communicable diseases
• A communicable disease is an illness due to a
specific infectious (biological) agent or its toxic
products capable of being directly or indirectly
transmitted from man to man, from animal to
man, from animal to animal, or from the
environment (through air, water, food, etc..) to
man.

43. Importance of Studying Communicable
Diseases Epidemiology
• Changes of the pattern of infectious diseases
• Discovery of new infections
• The possibility that some chronic diseases
have an infective origin.

44. Terminology and Definitions
•
•
•
•
•
•
•
•
•
•
Infection
Contamination
Infestation
Host
Contagious disease
Epidemic
Endemic
Hyperendemic
Holoendemic
Pandemic
• Exotic
• Sporadic
• Zoonosis, epizootic and
enzootic
• Nosocomial infection
• Opportunistic infection
• Eradication
• Elimination

45. Infection
• Infection is the entry and development or
multiplication of an infectious agent in the body of
man or animals.
• An infection does not always cause illness.
• There are several levels of infection (Gradients of
infection):
–
–
–
–
Colonization (S. aureus in skin and normal nasopharynx)
Subclinical or inapparent infection (polio)
Latent infection (virus of herpes simplex)
Manifest or clinical infection

46. Contamination
• The presence of an infectious agent on a body
surface, on or in clothes, beddings, toys,
surgical instruments or dressings, or other
articles or substances including water and
food.

47. Infestation
• It is the lodgment, development and
reproduction of arthropods on the surface of
the body or in the clothing, e.g. lice, itch mite.
This term could be also used to describe the
invasion of the gut by parasitic worms, e.g.
ascariasis.

48. Host
• A person or an animal that affords subsistence or
lodgement to an infectious agent under natural
conditions.
• Types include:
– an obligate host- the only host
– definitive (primary) host-attains maturity or passes
sexual stages
– intermediate host-passes asexual stage
– transport host-does not undergo development

49. Contagious disease
• A contagious disease is the one that is
transmitted through contact.
Examples - scabies, trachoma, STD.

50. Epidemic
• “The unusual occurrence in a community of
disease, specific health related behavior, or
other health related events clearly in excess of
expected occurrence”
• (epi= upon; demos= people)
• Epidemics can occur upon endemic states too.

51. Endemic
• It refers to the constant presence of a disease
or infectious agent within a given geographic
area or population group. It is the usual or
expected frequency of disease within a
population.
• (En = in; demos = people)

52. Endemic - Epidemic - Pandemic
R>1
R=1
R<1
Time
 Endemic
 Transmission occur, but the number of cases remains
constant
 Epidemic
 The number of cases increases
 Pandemic
 When epidemics occur at several continents – global
epidemic
(www)

53. Number of Cases of a Disease
Endemic vs Epidemic
Endemic
Time
Epidemic

54. Hyperendemic and holoendemic
• The term “hyperendemic” expresses that the disease is
constantly present at high incidence and/or prevalence rate
and affects all age groups equally.
• The term “holoendemic” expresses a high level of infection
beginning early in life and affecting most of the child
population, leading to a state of equilibrium such that the
adult population shows evidence of the disease much less
commonly than do the children (e.g. malaria)

55. Sporadic
• The word sporadic means “scattered about”.
• Cases - irregularly, haphazardly and generally
infrequently.
• Cases - few and separated widely in time and place
e.g. polio, meningococcal meningitis, tetanus….
• May be starting point of an epidemic

56. Pandemic and Exotic
• An epidemic usually affecting a large proportion of the
population, occuring over a wide geographic area such as a
section of a nation, the entire nation, a continent or the world,
e.g. Influenza pandemics(1918,1957 & 2009).
• Exotic diseases are those which are imported into a country in
which they do not otherwise occur, as for e.g., rabies in the UK,
Yellow fever in India, CCHF

57. Zoonosis, Epizootic and Enzootic
• Zoonosis is an infection that is transmissible under
natural conditions from vertebrate animals to man, e.g.
rabies, plague, bovine tuberculosis
– Anthropozoonoses e.g., Rabies
– Zooanthroponoses e.g., Human TB in cattle
– Amphixenosis e.g., T.cruzi
• An Epizootic is an outbreak (epidemic) of disease in an
animal population, e.g. Rift valley fever, Anthrax.
• An Enzootic is an endemic occurring in animals, e.g.
Bovine TB.

58. Nosocomial infections
• Nosocomial (hospital acquired)
infection is an infection
originating in a patient while in a
hospital or another health care
facility. It has to be a new
disorder unrelated to the
patient’s primary condition. E.g.,
infection of surgical wounds,
hepatitis B and urinary tract
infections.

59. Opportunistic infection
• This is infection by organisms that take the
opportunity provided by a defect in host
defense (e.g. immunity) to infect the host and
thus cause disease.
• E.g., opportunistic infections are very common
in AIDS. Organisms include Herpes simplex,
cytomegalovirus, M. tuberculosis etc.

60. Iatrogenic (Physician induced) Disease
• Any untoward or 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., reaction to penicillin, hepatitis B infection
following blood transfusion.

61. Eradication
• Termination of all transmission of infection by
the extermination of the infectious agent
through surveillance and containment.
Eradication is an absolute process, an “all or
none” phenomenon, restricted to termination
of infection from the whole world.

62. Elimination
• The term elimination is sometimes used to
describe eradication of a disease from a large
geographic region. Disease which are
amenable to elimination in the meantime are
polio, measles, leprosy and diphtheria.

63. Dynamics of disease Transmission
(Chain of Infection)
I
Source or Reservoir
II
Modes of transmission
III
Susceptible host

64. (I): Source or 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.
• The reservoir is “any person, animal, arthropod, plant, soil, or
substance, or a combination of these, in which an infectious
agent normally lives and multiplies, on which it depends
primarily for survival, and where it reproduces itself in such a
manner that it can be transmitted to a susceptible host.
• It is the natural habitat of the infectious agent.”

65. Types of reservoirs
Reservoir
Human
reservoir
Animal
reservoir
Non-living
reservoir

66. Human Reservoir
1. AIDS (HIV infection)
2. Syphilis
3. Gonorrhea
4. Shigellosis
5. Typhoid fever
6. Hepatitis-B virus
7. Herpes simplex virus

67. Animal Reservoir (Zoonoses)
1. Nontyphoidal salmonellosis
2. Brucellosis
3. Anthrax
4. Listeriosis
5. Viral encephalitis (SLE,WEE, CEE)
6. Rabies
7. Plague

68. Soil Reservoir
1. Histoplasmosis
2. Coccidioidomycosis
3. Blastomycosis
4. Tetanus
5. Botulism

69. Water Reservoir
1. Pseudomonas infections
Sepsis, UTI, “hot tub” folliculitis
2. Legionnaires’ disease
3. Melioidosis

70. Human Reservoir
Human reservoir
Cases
•Primary case
•Index case
•Secondary cases
According to spectrum of disease:
•Clinical cases
(mild/severe-typical/atypical)
•Sub-clinical cases
•Latent infection cases
Type:
•Incubatory
•Convalescent
•Healthy
Duration:
•Temporary
•Chronic
Carriers
Portal of exit:
•Urinary
•Intestinal
•Respiratory
•others

71. Exposure to Infectious Agents
No infection
Death
Clinical
Carrier
Sub-clinical
Immunity
Outcome
Carrier
No immunity

72. Cases
• A case is defined as “a person in the
population or study group identified as having
the particular disease, health disorder, or
condition under investigation”

73. Carriers
•
•
•
It occurs either due to inadequate treatment or immune
response, the disease agent is not completely eliminated,
leading to a carrier state.
It is “an infected person or animal that harbors a specific
infectious agent in the absence of discernible (visible)
clinical disease and serves as a potential source of infection
to others.
Three elements have to occur to form a carrier state:
1.
2.
3.
The presence of the disease agent in the body.
The absence of recognizable symptoms and signs of disease.
The shedding of disease agent in the discharge or excretions.

74. Animal reservoirs
• Zoonosis is an infection that is transmissible
under natural conditions from vertebrate
animals to man, e.g. rabies, plague, bovine
tuberculosis.
• There are over a 100 zoonotic diseases that
can be conveyed from animal to man.

75. (II): Modes of transmission
Mode of transmission
Direct
transmission
Direct contact
Droplet infection
Contact with soil
Inoculation into skin or mucosa
Trans-placental (vertical)
Indirect
transmission
Vehicle-borne
Vector-borne:
Mechanical
Biological
Air-borne
Propagative
Cyclo-propagative
Cyclo-developmental
Fomite-borne
Unclean hands &
fingers

76. Routes of transmission
Direct
Indirect
 Skin-skin
 Herpes type 1
 Mucous-mucous
 STI
 Across placenta
 toxoplasmosis
 Through breast milk
 HIV
 Sneeze-cough
 Influenza
 Food-borne
 Salmonella
 Water-borne
 Hepatitis A
 Vector-borne
 Malaria
 Air-borne
 Chickenpox
 Ting-borne
 Scarlatina
Exposure
 A relevant contact – depends on the agent
Skin, sexual intercourse, water contact, etc
(www)

77. Modes of Disease Transmission

78. • Propagative transmission(e.g. viruses, YF, WNV, EEE, etc.)
• Cyclo-developmental
(e.g. Wuchereria bancrofti-Bancroftian filariasis)
• Cyclo-propagative transmission-.
(e.g. malaria, Chagas)
10/15/2013
78

79. (III): Susceptible host
•
•
An infectious agent seeks a susceptible host aiming
“successful parasitism”.
Four stages are required for successful parasitism:
1.
2.
3.
4.
Portal of entry
Site of election inside the body
Portal of exit
Survival in external environment

80. Incubation and Latent periods
• Incubation period: time from exposure to
development of disease. In other words, the time
interval between invasion by an infectious agent
and the appearance of the first sign or symptom
of the disease in question.
• Latent period: the period between exposure and
the onset of infectiousness (this may be shorter
or longer than the incubation period).

81. Serial interval, Generation time and Infectious period
• Serial interval: Gap in time between the onset
of the primary and the secondary cases
• Generation time : Interval between receipt of
infection and maximal infectivity of the host
• Infectious (communicable) period: length of
time an infectious agent can be transmitted
directly or indirectly from an infected person
to another person, from an infected animal to
man or from an infected person to animal.

82. • Index Case
– Person that comes to the attention of
public health authorities
• Primary Case
– Person who acquires the disease from an
exposure
• Secondary Case
– Person who acquires the disease from an
exposure to the primary case
– Secondary attack rate

83. Secondary attack rate
• The number of exposed persons developing
the disease within the range of the incubation
period, following exposure to the primary
case.
• SAR =
No. of exposed persons developing the disease
within the range of incubation period
Total no. of exposed / susceptible contacts
X 100

84. Virulence and Case Fatality Rate
• Virulence
– Degree of pathogenicity; the disease evoking power of a microorganism in a given host.
– Numerically expressed as the ratio of the number of cases of
overt infection to the total number infected.
– When death is the only criterion of severity, this is the case
fatality rate.
• Case fatality rate
– Proportion of infected individuals who die of the infection. This is
a function of the severity of the infection.

85. Case Fatality Rate
Case fatality rate (%)
=
Number of deaths due to disease
x 100
Number of cases of disease

86. Host defences
• Local
• Systemic
Active Immunity
•Humoral
•Cellular
•Combination
• Specific
Passive Immunity
• Non Specific
•Normal human Ig
•Specific Human Ig
•Animal Antitoxins or Antisera

87. Herd Immunity
• The level of resistance of a community or group of people to
a particular disease.
• Provides an immunological barrier to spread of disease in
the human herd.
• If herd immunity sufficiently high, the occurrence of
epidemic is unlikely.
• If high level of immunity is achieved and maintained to a
point where the susceptible persons are reduced to a small
proportion, it may even lead to elimination of a disease e.g.,
Polio.
• Herd immunity does not protect against Tetanus.

89. THANK YOU
THANK YOU

92. Infectious Disease Process
Direct tissue invasion
Toxins
Persistent or latent infection
Altered susceptibility to drugs
Immune suppression
Immune activation (cytokine storm)

93. • Microbial pathogenesis- process of causing
disease
• Colonization - presence of microbes at site
of body
– Does not imply tissue damage or disease
symptoms
– Does imply invasion of site and multiplication

94. Timeline for Infection
Dynamics of
infectiousness
Latent
period
Infectious
period
Non-infectious
Susceptible
Time
Dynamics of
disease
Incubation
period
Symptomatic
period
Non-diseased
Susceptible
Time
(www)

95. Transmission
Cases
 Index – the first case identified
 Primary – the case that brings the infection into a population
 Secondary – infected by a primary case
 Tertiary – infected by a secondary case
T
S
P
Susceptible
Immune
Sub-clinical
S
S
T
Clinical
(www)

96. After invasion: the effective
reproduction number, R(t)
• As pathogen invades, the
number of susceptibles declines
through recovery (or death)
• Eventually, insufficient
susceptibles to maintain chains
of transmission
• On average each infectious
person infects < 1 other,
epidemic dies out
Initial invasion, R(t) = R0
Peak of epidemic R(t) = 1

97. Changes to R(t), over an epidemic
1200
number
1000
800
Susceptible
Incident cases
Recovered
R=1
600
R<1
R>1
400
R=R0
200
0
0
0.05
0.1
time
0.15
0.2

98. Reproductive Number, R0
• Useful summary statistic
• Definition: the average number of secondary
cases a typical infectious individual will cause
in a completely susceptible population
• Measure of the intrinsic potential for an
infectious agent to spread
(www)

99. Reproductive Number, R0
A measure of the potential for transmission
The basic reproductive number, R0, the mean number of individuals directly
infected by an infectious case through the total infectious period, when
introduced to a susceptible population
probability of transmission per contact
R0 = p • c • d
duration of infectiousness
contacts per unit time
(www)

100. Infection will …..
if
R < 1 --------- disappear,
if
R = 1 --------- become endemic,
if
R > 1 --------- become epidemic,

101. Reproductive Number, R0
• If R0 < 1 then infection cannot invade a
population
– implications: infection control mechanisms
unnecessary (therefore not cost-effective)
• If R0 > 1 then (on average) the pathogen will
invade that population
– implications: control measure necessary to
prevent (delay) an epidemic

102. Reproductive Number, R0
Use in STI Control
R0 = p • c • d
p
c
condoms, acyclovir, zidovudine
health education, negotiating skills
D
case ascertainment (screening,
partner notification), treatment,
compliance, health seeking behaviour,
accessibility of services
(www)

103. What determines R0 ?
p, transmission probability per exposure – depends on the
infection
 HIV, p(hand shake)=0, p(transfusion)=1, p(sex)=0.001
 interventions often aim at reducing p
 use gloves, screene blood, condoms
c, number of contacts per time unit – relevant contact
depends on infection
 same room, within sneezing distance, skin contact,
 interventions often aim at reducing c
 Isolation, sexual abstinence
d, duration of infectious period
 may be reduced by medical interventions (TB, but not
salmonella)
(www)

104. Immunity – herd immunity
If R0 is the mean number of secondary cases in a susceptible population, then
R is the mean number of secondary cases in a population where a proportion, p, are
immune
R = R0 – (p • R0)
What proportion needs to be immune to prevent epidemics?
If R0 is 2, then R < 1 if the proportion of immune, p, is > 0.50
If R0 is 4, then R < 1 if the proportion of immune, p, is > 0.75
If the mean number of secondary cases should be < 1, then
R0 – (p • R0) < 1
p > (R0 – 1)/ R0 = 1 – 1/ R0
 If R0 =15, how large will p need to be to avoid an epidemic?
p > 1-1/15 = 0.94
The higher R0, the higher proportion of immune required for herd immunity
(www)

105. Vaccination coverage
required for elimination
Critical proportion, Pc
100%
Pc = 1-1/Ro
80%
60%
40%
20%
rubella
0%
0
2
4
6
measles
8 10 12 14 16 18 20
Basic reproduction number, Ro