Infectious Disease
Epidemiology
Principles of Epidemiology
Lecture 7
Dona Schneider, PhD, MPH, FACE
Concepts in Infectious Epidemiology

Agent, host, environment

Classification of human infections by
modes of transportation

Incubation period

Spectrum of disease

Herd immunity
Epidemiology (Schneider)
Major Factors Contributing to the
Emergence of Infectious Diseases
 Human demographics and behavior
 Technology and industry
 Economic development and land use
 International travel and commerce
 Microbial adaptation and change
 Breakdown of public health measures
Epidemiology (Schneider)
Epidemiologic Triad Concepts

Infectivity – ability to invade a host
(# infected / # susceptible) X 100

Pathogenicity – ability to cause disease
(# with clinical disease / # of infected) X 100

Virulence – ability to cause death
(# of deaths / # with disease (cases)) X 100

All are dependent upon the condition of
the host
-
Immunity (active, passive)
-
Nutrition
-
Sleep
-
Hygiene
Mode of Transmission
 Person-to-person (respiratory, orogenital, skin)
 Examples: HIV, measles
 Vector (animals, insects)
 Examples: rabies, yellow fever
 Common vehicle (food, water)
 Examples: salmonellosis
 Mechanical vectors (personal effects) such as
doorknobs, or toothbrushes are called FOMITES
Epidemiology (Schneider)
Classification by Mode of Transmission

Dynamics of Spread through Human
Populations


Spread by a common vehicle

Ingestion
Salmonellosis

Inhalation
Legionellosis

Inoculation
Hepatitis
Propagation by serial transfer from host to host

Respiratory
Measles

Anal-oral
Shigellosis

Genital
Syphilis
Epidemiology (Schneider)

Principle Reservoir of Infection
 Man
Infectious hepatitis
 Other vertebrates (zoonoses) Tularemia
 Agent free-living
Histoplasmosis

Portal of Entry/Exit in Human Host
 Upper respiratory tract
Diphtheria
 Lower respiratory tract
Tuberculosis
 Gastrointestinal tract
Typhoid fever
 Genitourinary tract
Gonorrhea
 Conjunctiva
Trachoma
 Percutaneous
Leptospirosis
 Percutaneous (bite of arthropod) Yellow fever
Epidemiology (Schneider)

Cycles of Infectious Agent in Nature
 Man-man
Influenza
 Man-arthropod-man
Malaria
 Vertebrate-vertebrate-man
Psittacosis
 Vertebrate-arthropod-vertebrate-man

Viral encephalitis
Complex Cycles
Helminth infections
Epidemiology (Schneider)
River blindness
Incubation Period

The interval between the time of contact
and/or entry of the agent and onset of
illness (latency period)

The time required for the multiplication of
microorganisms within the host up to a
threshold where the parasitic population
is large enough to produce symptoms
Epidemiology (Schneider)

Each infectious disease has a characteristic
incubation period, dependent upon the rate of
growth of the organism in the host and
Dosage of the infectious agent
Portal of entry
Immune response of the host

Because of the interplay of these factors,
incubation period will vary among individuals
For groups of cases, the distribution will be a
curve with cases with longer incubation periods
creating a right skew
Epidemiology (Schneider)
Spectrum of Disease


Exposure

Subclinical manifestations

Pathological changes
Symptoms

Clinical illness

Time of diagnosis

Death

Whether a person passes through all these
stages will depend upon infection and
prevention, detection and therapeutic measures
Epidemiology (Schneider)
Iceberg Concept of Infection
CELL RESPONSE HOST RESPONSE
Lysis of cell
Discernable
Cell transformation
effect
or
Cell dysfunction
Incomplete viral
Below visual maturation
change
Exposure
without cell entry
Fatal
Clinical and
severe disease
Clinical
Disease
Moderate severity
Mild Illness
Infection without
clinical illness Subclinical
Disease
Exposure
without infection
Spectrum of Disease (cont.)

Example

90% of measles cases exhibit clinical
symptoms

66% of mumps cases exhibit clinical
symptoms

<10% of poliomyelitis cases exhibit
clinical symptoms
Inapparent infections play a role in
transmission. These are distinguished from
latent infections where the agent is not shed
Epidemiology (Schneider)
Subclinical/Clinical Ratio for Viral Infections
Virus
Clinical feature Age at infection Estimated ratio Clinical cases
+ 1000:1
Paralysis
Child
Polio
Epstein-Barr Mononucleosis 1 to 5 years
> 100:1
6 to 15 years
10:1 to 100:1
16 to 25 years
2:1 to 3:1
Hepatitis A
Icterus
< 5 years
20:1
5 to 9 years
11:1
10 to 15 years 7:1
Adult
1.5:1
Rubella
Rash
5 to 20 years
2:1
Influenza
Fever, cough Young adult
1.5:1
Measles
Rash, fever
5 to 20 years
1:99
CNS symptoms Any age
<1:10,000
Rabies
0.1% to 1.0%
1%
1% to 10%
50% to 75%
5%
10%
14%
80% to 95%
50%
60%
>99%
>>>>99%
Herd Immunity

The decreased probability that a group
will develop an epidemic because the
proportion of immune individuals reduces
the chance of contact between infected
and susceptible persons

The entire population does not have to be
immunized to prevent the occurrence of
an epidemic

Example: smallpox, measles
Epidemiology (Schneider)
Investigating an Epidemic
 Determine whether there is an
outbreak – an excess number of
cases from what would be expected
There
must be clarity in case definition
and diagnostic verification for each case
Investigating an Epidemic (cont.)
 Plot an epidemic curve (cases against
time)
 Calculate attack rates
 If there is no obvious commonality for the
outbreak, calculate attack rates based on
demographic variables (hepatitis in a community)
 If there is an obvious commonality for the
outbreak, calculate attack rates based on
exposure status (a church supper)
Epidemiology (Schneider)
Investigating an Epidemic (cont.)
 Determine the source of the epidemic
 If there is no obvious commonality for the
outbreak, plot the geographic distribution of
cases by residence/work/school/location to
reduce common exposures
 If there is an obvious commonality for the
outbreak, identify the most likely cause and
investigate the source to prevent future
outbreaks
Epidemiology (Schneider)

Index Case



Person that comes to the attention of
public health authorities
Primary Case

Person who acquires the disease from an
exposure

Attack rate
Secondary Case

Person who acquires the disease from an
exposure to the primary case

Secondary attack rate
Epidemiology (Schneider)
Calculation of Attack Rate for Food X
Ate the food (exposed)
Did not eat the food (not exposed)
Ill
Well Total Attack Ill
Well Total Attack
Rate
Rate
10
3
13
76%
7
4
11
64%
Attack Rate = Ill / (Ill + Well) x 100 during
a time period
Attack rate = (10/13) x 100 = 76%
( 7/11) x 100 = 64%
RR = 75/64 = 1.2
Epidemiology (Schneider)
Secondary Attack Rate
Secondary Total number of cases – initial case(s)
x 100
attack rate =
Number of susceptible persons
(%)
in the group – initial case(s)

Used to estimate to the spread of disease in a
family, household, dorm or other group
environment.

Measures the infectivity of the agent and the
effects of prophylactic agents (e.g. vaccine)
Epidemiology (Schneider)
Mumps experience of 390 families exposed
to a primary case within the family
Population
Cases
No. susceptible before
Primary
primary cases occurred
Age in
years
Total
2-4
300
250
100
50
5-9
450
204
87
10-19
152
420
84
25
15
Secondary attack rate 2-4 years old =
(150-100)/(250-100) x 100 = 33%
Epidemiology (Schneider)
Secondary
Case Fatality Rate
Number of deaths due to disease X
Case fatality
=
Number of cases of disease X
rate (%)

Reflects the fatal outcome
(deadliness) of a disease, which is
affected by efficacy of treatment
Epidemiology (Schneider)
x 100
Assume a population of 1000 people. In one year,
20 are sick with cholera and 6 die from the disease.
The cause-specific mortality rate in that year from cholera =
6
1000
= 0.006 = 0.6%
The case-fatality rate from cholera =
6 = 0.3 = 30%
20
Epidemiology (Schneider)