CHAPTER 25
Epidemiology
Principles of Epidemiology
The Science of Epidemiology
• Epidemiology is the study of disease in
populations. To understand infectious disease,
the epidemiologist studies the interactions of
the pathogen with the host population.
• Epidemiology is the study of disease in
populations. To understand infectious disease,
the epidemiologist studies the interactions of
the pathogen with the host population.
• Figure 25.1 shows deaths due to infectious disease
in the United States.
• A well-adapted pathogen lives in balance
with its host, taking what it needs for
existence and causing only a minimum of
harm. Such pathogens may cause a chronic,
or long-term, infection in the host.
• Pathogens can be selective forces in the
evolution of the host, just as hosts are
selective forces in the evolution of pathogens.
Such pathogens often cause acute infections,
characterized by rapid and dramatic onset and,
often, rapid recovery.
The Vocabulary of Epidemiology
• The prevalence of a disease in a population is
defined as the proportion of diseased individuals in
a population in a given time period. The incidence
of a disease is the number of cases of an individual
disease in a population in a given time period.
• An endemic disease is constantly present at
low incidence in a specific population. In
epidemics, an unusually high incidence of
disease occurs in a specific population. A
pandemic is a worldwide epidemic.
• Figure 25.2 shows worldwide incidence of
endemic, epidemic, and pandemic disease.
• Sporadic cases of disease may occur when individual
cases are recorded in geographically separated areas,
implying that the incidents are not related.
• A disease outbreak occurs when a number of cases are
observed, usually in a relatively short period of time, in
an area previously experiencing only sporadic cases of
the disease.
• Finally, diseased individuals who show no symptoms or
only mild symptoms have subclinical infections.
Subclinically infected individuals are frequently
identified as carriers of a particular disease.
• Infectious diseases cause morbidity (illness) and may
cause mortality (death). An infectious disease follows a
predictable clinical pattern in the host.
• Table 25.1 shows worldwide deaths due to
infectious diseases in 2002.
Disease Reservoirs and
Epidemics
• To understand how diseases spread, the
pathogen reservoir must be known.
• Some pathogens exist in soil, water, or animals.
Others exist only in humans and are maintained
solely by person-to-person contact. An
understanding of disease carriers and pathogen life
cycles is critical for controlling disease (Table 25.2).
Infectious Disease Transmission
• A pathogen can be transmitted directly from
one host to another, or indirectly by living
vectors or inanimate objects (fomites) and
common vehicles such as food and water.
• Environmental factors may play a role in
survival of a pathogen, and such variables as
weather patterns may influence exposure to a
pathogen (Figure 25.3).
• A common-source epidemic is one resulting
from infection of a large number of people
from a single contaminated source.
• A host-to-host epidemic results from
person-to-person contact and is characterized
by a gradual rise and fall in number of cases
(Figure 25.4).
The Host Community
• Hosts and pathogens coevolve with time
and arrive at a steady state that favors the
continued survival of both (Figure 25.5).
• With herd immunity, a large fraction of a population is immune to a
given disease, and it is difficult for the disease to spread (Figure 25.6).
• Disease cycles occur when a large,
recurring, nonimmune population such as
children entering school is exposed to a
pathogen.
Current Epidemics
The AIDS Pandemic
• AIDS is one of the most thoroughly studied
disease pandemics. AIDS will continue to be a
major public health problem, especially in
developing countries.
• There is no effective cure or immunization
to prevent AIDS, although we now know a
great deal about its pathology and spread.
• Table 25.3 shows the incidence of
HIV/AIDS infections worldwide in 2003.
• Figure 25.7 shows annual diagnosed cases of
AIDS since 1981 in the United States.
• The distribution of AIDS cases by risk group and sex in adolescents and
adults in the United States for the year 2002 is shown in Figure 25.8.
Hospital-Acquired
(Nosocomial) Infections
• Many common microorganisms have the
potential to be pathogens in a hospital
environment. This is called nosocomial
infection.
• Hospital patients are unusually susceptible
to infectious disease and are exposed to a
variety of infectious agents, including
opportunistic pathogens, in the hospital
environment. Treatment of these infections is
complicated by antibiotic resistance.
• Table 25.4 gives the number of intensive care unit nosocomial
infections in the United States, by site and organism.
Severe Acute Respiratory Syndrome
• Zoonosis is infection with a disease that
occurs primarily in animals but can be
transferred to humans.
• Infection with a novel zoonotic virus,
SARS-CoV, causes severe acute respiratory
syndrome (SARS). Person-to-person spread is
by respiratory means. Control of this high
mortality virus is through rapid diagnosis and
isolation of victims.
Epidemiology and Public
Health
Public Health Measures for the
Control of Disease
• Food and water purity regulations, vector
control, immunization, quarantine, disease
surveillance (Table 25.5), and pathogen
eradication are public health measures that
play a major role in reduction of disease
incidence.
• The Centers for Disease Control and
Prevention (CDC) in the United States,
through the National Center for Infectious
Diseases (NCID), operates a number of
surveillance programs (Table 25.6).
Global Health Considerations
• Infectious diseases account for nearly 30%
of all worldwide mortality. Most infectious
diseases occur in developing countries.
• Figure 25.10 indicates that most African
deaths result from infectious diseases;
whereas in the Americas, cancer and
cardiovascular diseases are the leading causes
of mortality.
• Travelers to endemic disease areas should be immunized
when possible and should take appropriate precautions to
prevent infection (Table 25.7).
Emerging and Reemerging
Infectious Diseases
• Changes in host, vector, or pathogen
conditions, whether natural or artificial, can
result in conditions that encourage the
explosive emergence or reemergence of
certain infectious diseases.
• Recent dramatic examples of global
emerging and reemerging infection are
shown in Figure 25.11. Table 25.8 shows
emerging and reemerging epidemic infectious
diseases.
• Bacterial genetic mechanisms can enhance
virulence and promoting emergence of new
epidemics. One group of virulence-enhancing
mechanisms are the mobile genetic elements,
bacteriophages, plasmids and transposons.
• Table 25.9 lists some virulence factors
carried on these mobile genetic elements that
contribute to pathogen emergence.
• Global surveillance and intervention
programs must be developed to prevent new
epidemics and pandemics.
Biological Warfare and
Biological Weapons
• Biological warfare is the use of biological
agents to incapacitate or kill a military or
civilian population in an act of war or
terrorism.
• Bioterrorism is a threat in a world of rapid
international travel and easily accessible
technical information. Biological agents can
be used as weapons by contemporary military
forces or by terrorist groups.
• Aerosols or common sources such as food
and water are the most likely mode of
inoculation. Commonly considered biological
weapons agents are listed in Table 25.10.
• Prevention and containment measures rely
on a well-prepared public health
infrastructure.
Anthrax as a Biological Weapon
• Bacillus anthracis has emerged as an
important pathogen because of its use as a
bioweapon.
• Highly infective weaponized endospore
preparations have been used as bioterror
agents. Inhalation anthrax has a fatality rate of
about 90% in untreated individuals.
• Effective treatment relies on timely
observation and diagnosis of symptoms.
Treatment does not guarantee survival of
those infected by inhalation anthrax.