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Chapter 8
Infectious Diseases and Outbreak
Investigation
Learning Objectives
• Describe modes of transmission of
communicable diseases
• Name three microbial agents associated with
infectious diseases
• Describe the epidemiology of two infectious
diseases
• State procedures for investigating infectious
disease outbreaks
Introduction
• Infectious diseases are important causes of
morbidity and mortality in the U.S. and
worldwide.
– Category of influenza and pneumonia was eighth
leading cause of death in U.S. in 2005.
– Infectious/parasitic diseases account for almost
20% of mortality worldwide.
– Respiratory infections account for 7% of mortality
worldwide.
Infectious (Communicable)
Disease
• “An illness due to a specific infectious agent or
its toxic products that arises through
transmission of that agent or its products from
an infected person, animal, or reservoir to a
susceptible host, either directly or indirectly
through an intermediate plant or animal host,
vector, or the inanimate environment.”
Parasitic Disease
• An infection caused by a parasite, which “…is
an organism that lives on or in a host organism
and gets its food from or at the expense of its
host.”
• Example: amebiasis
Epidemiologic Triangle
• One of the long-standing models used to
describe the etiology of infectious diseases
• Includes three major factors: agent, host, and
environment
Agent
• “A factor—such as a microorganism, chemical
substance, or form of radiation—whose
presence, excessive presence, or (in deficiency
diseases) relative absence is essential for the
occurrence of a disease….”
Host
• “A person or other living animal, including
birds and arthropods, that affords subsistence
or lodgment to an infectious agent under
natural conditions.”
Environment
• The domain in which disease-causing agents
may exist, survive, or originate
The Epidemiologic Triangle
Source: Reprinted from Friis RH, Sellers TA. Epidemiology for Public Health Practice. 4th ed. Sudbury,
Massachusetts: Jones and Bartlett Publishers; 2009:439.
Infectious Disease Agents
• With respect to infectious and communicable
diseases, agents include specific microbes and
vectors involved in the cycle of disease
transmission.
– Examples of infectious agents: bacteria, rickettsia,
viruses, fungi, parasites, and prions
Infectivity
• The capacity of an agent to enter and multiply
in a susceptible host and thus produce
infection or disease
Virulence
• Refers to the severity of the disease produced,
i.e., whether the disease has severe clinical
manifestations or is fatal in a large number of
cases.
Toxin
• Some infectious disease agents, instead of
acting directly, produce a toxin that causes
illness.
• A toxin usually refers to a toxic substance
made by living organisms.
– Examples of illness: foodborne intoxications such
as botulism.
Four infectious disease agents
Upper left,
bacillus
anthracis
bacteria
Lower left,
dermatophyti
c fungus
(causes
ringworm
infections of
the skin and
fungal
infections of
the nail bed
Upper right,
herpes simplex
virions
Lower right,
Cryptosporidium
parvum oocysts
Source: Reprinted from Centers for Disease Control and Prevention. Public Health Image Library, ID# 10123 (upper left); ID# 10230
(upper right); ID# 4207 (lower left); ID# 7829 (lower right). Available at: http://phil.cdc.gov/phil/details.asp. Accessed July 3, 2008.
Host Characteristics
• Immunity
• Other factors related to host responses:
– Incubation period
– Subclinical illness
– Generation time
– Carrier status
Immunity
• Refers to the host’s ability to resist infection by
the agent.
• May be either active or passive
Active Immunity
• Immunity that the host has developed as a
result of natural infection with a microbial
agent
• Also can be acquired from an injection of a
vaccine that contains an antigen
– Antigen: A substance that stimulates antibody
formation, e.g., a microbial agent
• Usually of long duration (measured in years)
Passive Immunity
• Immunity acquired from antibodies produced
by another person or animal
• Example 1: newborn infant’s natural immunity
conferred transplacentally from its mother
• Example 2: immunity conferred by injections
of antibodies contained in immune serums
• Usually of short duration (measured in days to
months)
Herd Immunity
• The resistance of an entire community to an
infectious agent as a result of the immunity of
a large proportion of individuals in that
community to the agent
Incubation Period
• Time interval between invasion by an
infectious agent and the appearance of the first
sign or symptom of the disease
Subclinical Infection
• An infection that does not show obvious
clinical signs or symptoms
• Also called an inapparent infection
• Example: hepatitis A infections among
children
Generation Time
• The time interval between lodgment of an
infectious agent in a host and the maximal
communicability of the host
Carrier
• “A person or animal that harbors a specific
infectious agent without discernible clinical
disease, and which serves as a potential source
of infection.”
Index Case
• Used in an epidemiologic investigation of a
disease outbreak to denote the first case of a
disease to come to the attention of authorities
Environment and Infectious
Diseases
• The external environment comprises these
components:
– Physical
– Climatologic
– Biologic
– Social
– Economic
Endemic
• Refers to a infectious disease agent that is
habitually present in an environment (either
geographic or population group)
• Example: Plague is endemic among certain
species of rodents in the western U.S.
Reservoir
• A place where infectious agents normally live
and multiply
• Can be human beings, animals, insects, soils,
or plants
Zoonosis
• “An infection or infectious agent transmissible
under natural conditions from vertebrate
animals to humans.”
Means of Transmission of
Infectious Disease Agents
• Direct transmission
– Person to person
• Indirect transmission
– Vehicle-borne infections
– Airborne infections
– Vector-borne infections
Portal of Exit
• For an infected person, a portal of exit is the
site from which the agent leaves that person’s
body.
• Examples:
– Respiratory passages
– Alimentary canal
– Genitourinary system
– Skin lesions
Person to Person
(Direct Transmission)
• “Direct and essentially immediate transfer of
infectious agents to a receptive portal of entry
through which human or animal infection may
take place….”
Portal of Entry
• Site where the agent enters the body
• Example: a skin wound
Indirect Transmission
• Involves intermediary sources of infection
such as vehicles, droplet nuclei (particles), and
vectors.
Vehicle-borne Infections
• These infections result from contact with
vehicles, which are contaminated, nonmoving
objects.
• Examples:
– Fomites
– Unsanitary food
– Impure water
Unsafe injection practices and circumstances
that likely resulted in transmission of
hepatitis C virus (HCV) at clinic A—Nevada,
2007
Source: Reprinted from Centers for Disease Control and Prevention. Acute hepatitis C virus infections attributed to unsafe injection
practices at an endoscopy clinic—Nevada, 2007. MMWR. 2008;57:516.
Fomite
• An inanimate object that carries infectious
disease agents
• Examples:
– Classroom doorknob
– Discarded tissues
Airborne Infections
• Involve the spread of droplet nuclei (particles)
that are present in the air.
• Example: infections caused by stirring up dust
that carries fungi or microbes
Vector-borne Infections
• A vector is an animate, living insect or animal
that is involved with the transmission of
disease agents.
• Transmission of an infectious disease agent
may happen when the vector feeds on a
susceptible host.
Four vectors of infectious diseases
Upper left, a
female
louse
Lower left, a
tick
Upper right, a
female Aedes
aegypti
mosquito
acquiring a
blood meal
Lower right, a
Norway rat.
Source: Reprinted from Centers for Disease Control and Prevention. Public Health Image Library, ID# 9250 (upper left); ID# 9255
(upper right); ID# 9959 (lower left); ID# 5445 (lower right). Available at: http://phil.cdc.gov/phil/details.asp. Accessed July 3, 2008.
Examples of Significant
Infectious Diseases
•
•
•
•
•
•
•
•
Sexually transmitted diseases
Foodborne diseases
Waterborne diseases
Vector-borne diseases
Vaccine-preventable diseases
Zoonotic diseases
Emerging infections
Bioterrorism-related diseases
Sexually Transmitted Diseases
• HIV/AIDS
• Gonococcal infections
• Chlamydial genital infections
Foodborne Illness
• Biologic agents of foodborne illness include
bacteria, parasites, viruses, and prions (linked
to mad cow disease).
• Examples of bacterial agents include:
– Campylobacter
– Clostridium botulinum
– Salmonella
Source: Reprinted from Centers for Disease Control and Prevention. Summary of Notifiable Diseases—United States, 2006. MMWR.
2008;55:42.
Vector-borne Diseases
• Examples of diseases (name of vector in
parentheses) from Table 8-6:
– Bacterial diseases
• Lyme disease (tick)
– Arthropod-borne (arboviral) diseases
• Eastern equine encephalitis (mosquito)
– Parasitic diseases
• Malaria (mosquito)
Vaccine-Preventable Diseases
(VPDs)
• Conditions that can be prevented by
vaccination (immunization)
• Examples include:
– Diptheria
– Tetanus
– Whooping cough
– Hepatitis A and B
Zoonotic Diseases
• Diseases transmitted from vertebrate animals
to human beings
• Examples:
– Rabies
– Anthrax
– Avian influenza
– Hantavirus pulmonary syndrome
– Toxoplasmosis
– Tularemia (rabbit fever)
Emerging Infectious Diseases
(Emerging Infections)
• “An infectious disease that has newly appeared
in a population or that has been known for
some time but is rapidly increasing in
incidence or geographic range.”
• Examples: hepatitis C, E. coli O157:H7.
Bioterrorism-Related Diseases
• Bioterrorism attack: “…the deliberate release
of viruses, bacteria, or other germs (agents)
used to cause illness or death in people,
animals, or plants….”
• Example: anthrax
Methods of Outbreak
Investigation
•
•
•
•
•
•
•
Clinical observations
Epidemic curve
Incubation period
Attack rate
Case mapping (used by John Snow)
Hypothesis formulation and confirmation
Draw conclusions
Clinical Observations
• The pattern of symptoms suggests possible infectious
agents.
• Possible symptoms of an outbreak include:
– Fever
– Nausea
– Diarrhea
– Vomiting
– Headache
– Rashes
– Stomach pain
Epidemic Curve
• “A graphic plotting of the distribution of cases
by time of onset.”
• May reflect a common-source epidemic or a
point-source epidemic.
Common-Source Epidemic
• An “Outbreak due to exposure of a group of
persons to a noxious influence that is common
to the individuals in the group.”
Point-Source Epidemic
• A type of common-source epidemic that occurs
“When the exposure is brief and essentially
simultaneous, [and] the resultant cases all
develop within one incubation period of the
disease…”
Incubation Period
• In conjunction with information about
symptoms, provides clues to possible
infectious disease agents
• Example: The incubation period for foodborne
illness caused by Salmonella ranges from 6 to
72 hours.
Attack Rate
• Used frequently to describe the occurrence of
foodborne illness, infectious diseases, and
other acute epidemics
• Formula: Attack rate = Ill/(Ill + Well) X 100
during a time period
Final Steps in Outbreak
Investigation
• Case mapping: Some investigations may use
computer software to show the location of
cases.
• Hypotheses: Using the information that has
been gathered, the epidemiologist may
formulate a hypothesis regarding the causative
agent.
• Draw a conclusion: Plan for the prevention of
future outbreaks.
Conclusion
• Infectious diseases remain a threat for the
following reasons:
– New types of diseases are constantly evolving and
imperiling public health.
– Outbreaks caused by bioterrorism are a potential
threat.
– Some infectious disease agents mutate into forms
that resist conventional antibiotic treatment.