Infectious Disease
Epidemiology
Casey E. Barton, M.S.
July 26, 2001
REMINDERS!!!!!!!!!!

TODAY by 5pm
– Submit all outstanding homeworks and redos

July 31st
– Teaching-Learning Exam
– Course Evaluations
– Tutorial after class

August 2nd
– FINAL EXAM
– Last day of class!!!!!!!
Top 10 Reported Infectious Diseases 1999
USA
Chlamydia
Gonorrhea
AIDS
Salmonellosis
Hepatitis A
Shigellosis
Tuberculosis
Lyme Disease
Hepatitis B
Syphilis
TEXAS
Chlamydia
Gonorrhea
Chickenpox
HIV
AIDS
Hepatitis A
Shigellosis
Salmonellosis
Tuberculosis
Campylobacteriosis
Epidemiologic Triangle
HOST
AGENT
ENVIRONMENT
AGENTS
An Agent MUST be present for an
infectious disease to develop
 Epidemiologists describe:

– Frequency of disease
– Mechanisms of transmission
– The causal agent
AGENTS
Bacteria
 Viruses and rickettsia
 Mycoses (fungi)
 Parasites

– Protozoa
– Helminths

Arthropods
AGENTS: Bacteria
Leading killers in 19th century
 Examples:

– Tuberculosis
– Salmonellosis
– Streptococcal infections

Problem: emerging bacteria strains
resistant to current antibiotics
AGENTS: Viruses and
Rickettsia

Viral infection include:
– Hepatitis A
– Herpes simplex
– Influenza
– HIV

Rickettsial infections include:
– Q fever
– Rocky Mountain Spotted Fever
Mycoses (Fungi)

Examples:
– Ringworm
– Aspergillosis
– Candidiasis
AGENTS: Parasites

Protozoa
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
Malaria
Cyclosporiasis
Giardiosis
Chagas’ Disease
Helminths (worms)
– Tapeworms
– Roundworms
– Schistosomiasis
AGENTS: Arthropods

Insect vectors including:
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–
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
Mosquitoes
Ticks
Flies
Mites
Other insects
Arthropod-borne diseases include:
–
–
–
–
RMSF
Malaria
Encephalitis
Lyme Disease
Modes of Transmission
Airborne or Contact
 Food- and Water-borne
 Blood-borne
 Sexually Transmitted
 Arthropod-borne
 Zoonotic

Means of Transmission
Portal of
Exit
Mode of
transmission
Portal of
Entry
Example of
Disease
Respiratory
secretions
Airborne droplets
Respiratory
tract
Common
cold,
measles
Feces
Water, food, flies
Alimentary
tract
Parasites,
Typhoid
Lesions,
exudate
Direct contact,
sexual contact
Skin, genital
membranes
Chlamydia,
gonorrhea
Conjunctival
exudate
Fomites, flies
Ocular
mucous
membranes
Trachoma
(blindness)
Blood
Blood-sucking
arthropods,
transfusions
skin
Malaria,
Encephalitis
Means of Transmission
Direct: person to person
 Indirect: through intermediate sources
such as vehicles, fomites or vectors
 Horizontal: transmission from one
person to another (direct or indirect)
 Vertical: mother to fetus either genetic
or across placenta

Characteristics of Infectious
Disease Agents
Infectivity
 Pathogenicity
 Virulence
 Toxigenicity
 Resistance
 Antigenicity

Infectivity
Capacity of the agent to enter and
multiply in a susceptible host and thus
produce infection or disease
 Examples: measles and polio
 Measure of Infectivity: Secondary Attack
Rate (SAR%)

Pathogenicity
Capacity of the agent to cause disease
in the infected host
 Example: measles
 Measure of pathogenicity: proportion of
individuals with clinically apparent
disease

Virulence

Severity of the disease
– i.e. whether clinical manifestations are
produced
Example: Rabies
 Measure of Virulence: Case Fatality
Rate (CFR)

Toxigenicity
Capacity of the agent to product a toxin
or poison
 Examples: botulism and shellfish
poisoning
 Disease is caused by toxins produced
by the agent, rather than by the actual
agent

Resistance
Ability of the agent to survive adverse
environmental conditions
 Examples: parasite with spores or cysts,
Hepatitis virus
 Resistance is also applied to the HOST

Antigenicity
Ability of the agent to induce antibody
production in the host
 Agents may or may not induce longterm immunity

– Examples: Measles re-infection is rare

Immunogenicity
– Related term
– refers to an infection’s ability to produce
specific immunity
Host

Subclinical infection  Active Disease

Host’s ability to fight off infectious agent
– Nonspecific defense mechanisms
– Disease-specific defense mechanisms
HOST: Non-specific defense
mechanisms
Age, Sex, Race, Genetics
 Religion, Customs, Occupation,
Nutritional status
 Skin, Mucosal surfaces, Saliva,
 High pH of gastric juices
 Immune system (phagocytes and
macrophages)

HOST: Disease-specific
defense mechanisms

Immunity: resistance of host to an agent
 Active: all or part of organism invokes an
immunologic response
– Natural: results from an infection
– Artificial: results from immunization

Passive: preformed antibody invokes short
term immunity
– Natural: antibodies cross the placenta
– Artificial: immuneglobulin after certain exposures
ENVIRONMENT
Physical: weather, temperature,
humidity, geologic formations, habitat
 Biological: population density, flora,
fauna, reservoirs
 Socioeconomical: behavioral,
personality, attitudinal, and cultural
characteristics of a group of people

Endemic vs.Epidemic
Endemic: constant presence of a
disease or agent in a defined
geographical area
 Epidemic: excessive occurrence of
disease (above endemic level) in a
defined geographical area

– 2 Types:
• Common Source
• Propogated Source
Epidemic

Common Source Epidemic: event or
exposure comes from a single source that all
persons in the group had a chance to
encounter
 Propogated Epidemic: caused by either direct
or indirect transmission of infectious disease
from one individual to another and can have
multiple sources from which disease can be
transmitted
Endemic versus Epidemic
ENDEMIC
Point EPIDEMIC
Propagating EPIDEMIC
NOTE: X axis = Time; Y axis = # new cases
Reservoir (Niche)
Fosters survival of infectious disease
agent
 Types of reservoirs

– Human
– Plant
– Animal
– Arthropod
– Physical environment
Inapparent infection
Subclinical infection: absence of
clinically obvious signs and symptoms
 Asymptomatic individuals can transmit
disease
 Carrier status
 Serologic evidence of infection

Incubation Period
Time interval between exposure to an
infectious agent and the appearance of
the first signs of disease
 During this time, the infectious agent
replicates within the host
 Can be hours, days, weeks, or years
 Applies only to clinically apparent cases
of disease

Generation Time

Time between lodgment of an infectious
agent in a host and the maximal
communicability of the host
 May or may not be equivalent to incubation
period
 Applies to both clinically apparent and
inapparent cases of disease
 Utilized for describing the spread of infectious
agents that have a large proportion of
subclinical cases
Herd Immunity

A population may become immune to an
infectious agent after a large proportion of
individuals have become immune
–

i.e. through past infections or vaccination
Can occur when immune persons prevent
the spread of disease to unimmunized
individuals and confers protection to the
population even though not every single
individual has been immunized
Effect of Herd Immunity on
Spread of Infection
(from Jekel et al. 1996, p. 12)
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Absence of Herd
Immunity
Presence of 50%
Herd Immunity
+
Iceberg Concept of Disease
Death
Visible Cases
Invisible
Cases
Clinical illness
Asymptomatic cases
Carriers
Zoonoses

World Health Organization (WHO):
– diseases and infections which are naturally
transmitted between vertebrate animals
and man
– Examples: Rabies, roundworms, Lyme
disease, Brucellosis, Cryptosporidiosis

Epizootic and enzootic but refers to diseases
in animals
Emerging Infectious Diseases
Changes in climate – global warming
 Human activities – farming,
reforestation
 Technologic changes – air travel, organ
transplantation
 Demographic changes – migration to
cities

Host Prevention
Immunity – immunizations
 Behavioral change
 Treatment of contact and carriers
 screening

Control

Environmental
– Personal hygiene
– Food protection and safety
– Water supplies
– Sanitation, regulation

Vectors
– Mosquitoes, insecticides
– Animal population control
Measures of Disease
Outbreaks
Attack Rate
 Secondary Attack Rate
 Case Fatality Rate

Attack Rate

Type of incidence rate
AR% =
# Ill___ * 100 during a time period
# Ill + # Well

Can calculate food or exposure specific
AR%
Secondary Attack Rate

Yields an index of the spread of disease
within a circumscribed unit, i.e. household,
dorm
 Index case: case that first comes to the
attention of public health authorities
 SAR% =
# new cases in group – initial cases * 100
# susceptible persons in group – initial cases
Case-Fatality Rate

Refers to the number of deaths caused by a
specific disease among those who have the
disease
 Provides index of deadliness of a particular
disease within a specific population
 CFR% = # deaths due to disease X * 100
# cases disease X
 DIFFERENT than crude death rate
Centers for Disease Control
and Prevention (CDC)

National Center for HIV,STD, and TB Prevention

National Center for Infectious Diseases
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AIDS, STD and TB Laboratory Research
Division of Bacterial and Mycotic Diseases
Division of Parasitic Diseases
Division of Vector-borne Infectious Diseases
Division of Viral and Rickettsial Diseases
National Immunization Program
Infectious Disease Information

Emerging Infectious Diseases Journal
– www.cdc.gov/ncidod/eid/

Morbidity and Mortality Weekly Report
(MMWR)
– www.cdc.gov/mmwr/

Program for monitoring Emerging Diseases
(Promed)
– www.promedmail.org/pls/promed/promed.home
THE END!!!
EXAMPLE CALCULATIONS
ATE
Food
DID NOT EAT
Ill Well Tot Ill
Well Tot
Chicken 32 8
40
20 40
60
Shrimp
40 30
70
10 20
30
Ice
Cream
48 12
60
2
40
38
Which food item appears to be the most probable
vehicle for the food-borne infection assocaited with
illness at this picnic?