Chapter 6 (108-112): Transmission of Infection, the Compromised Host,
Epidemiology and Diagnosing Infections
Incidence and prevalence
Epidemiology is the study of factors and mechanisms involved in the
frequency and spread of diseases or health-related problems. The
incidence of a disease is defined as the number of new cases contracted
within a set population in a specific period of time. This kind of
information can provide us with an indication of the spread of a particular
disease. Prevalence is defined as the total number of people infected
within a population at any given time. This means new cases and already
existing cases. Prevalence data can be used to measure how seriously and
for how long a particular disease affects the population.
Morbidity and mortality rates
The morbidity rate of a disease is the number of individuals affected by the disease during a
set period divided by the total population. The mortality rate is the number of deaths due to
a specific disease during a specific period divided by the total population.
When epidemiological studies are used to examine parameters such as particular geographic
areas and degree of harm caused by a disease, they classify diseases as sporadic, endemic,
epidemic and pandemic. Sporadic diseases occur in a random and unpredictable manner and
pose no threat to public health. Endemic refers to diseases that are constantly in the
population but in numbers too low to be a public health problem.
An epidemic occurs when the incidence of a disease suddenly becomes higher than the
normal expected number of individuals affected by the disease. An epidemic causes
morbidity and mortality rates to increase signaling a public health problem. There are several
factors that contribute to the development of an epidemic. For example, the large
populations that inhabit major cities allow the rapid spread of disease within a population.
Increased aging of a population leads to a higher percentage of people who are
immunologically or otherwise compromised, making them more susceptible to infection.
Lack of herd immunity as a result of incomplete or inadequate vaccination is another factor
in epidemics. Perhaps the most important factor is the ability to travel easily and transmit
diseases around the world. When a disease occurs in epidemic proportions throughout the
world, it is called a pandemic.
Epidemics are affected by the type of pathogen causing the disease and
mode of transmission. There are two types of epidemic:
common-source outbreaks and propagated epidemics.
1. Common-source outbreaks:
An epidemic that arises from contact with contaminated substances and
most commonly occurs when a water supply is contaminated with fecal
material or when food is improperly prepared or contaminated in
1
, processing. Although common-source outbreaks affect large numbers of people, they quickly
subside when the source has been resolved.
2. Propagated epidemic:
Results from amplification of the number of infected individuals as person-to-person contact
occurs. Propagated epidemics stay in the population for long periods and are more difficult
to deal with than common-source outbreaks.
Herd immunity
Herd immunity can be conferred through vaccination or can be developed after infection. For
example, in years past smallpox was responsible for millions of deaths. When a vaccination
against the disease was developed and given widely, the immunity of the population as a
whole to this infection increased. It does not mean that every individual in the population is
immune, but individual protection is conferred by limiting the number of possible hosts.
When the population, the herd, had become immune, the disease essentially disappeared
because there were insufficient susceptible hosts to maintain transmission of the pathogen.
Types of epidemiological study
Descriptive epidemiological studies are concerned with the physical aspects of patients and
the spread of disease. These studies include data on the number of cases and on which
segment of a population was affected. They also include location and time of infection, as
well as age, gender, race, marital status and occupation of those infected. Examination of this
type of information can allow epidemiologists to trace the outbreak of disease and possibly
identify the index case, which is the first person to have been infected.
In analytical epidemiological studies the focus is on establishing a cause-and-effect
relationship. These studies are done in conjunction with a control group, which is a
population known to be free of the disease. A retrospective analytical study is one in which
the records of patients who have already contracted the disease are studies. Using this type
of information, workers can take into account a wide variety of factors that preceded an
epidemic so that they can narrow down the potential cause. In prospective analytical studies
data are analyzed as they are collected. Prospective studies consider factors that occur as the
epidemic proceeds. Retrospective studies allow epidemiologists to develop and refine
models to get more accurate predictions for the future, which then allow taking preventative
or limiting measures for the benefit of everyone.
Health departments at the local and state level require that doctors and hospitals report
certain diseases. In addition, some diseases are classified as nationally notifiable, the ones
that are important to control and contain, which means they must be reported to national
centers such as the CDC, the clearing house for epidemiological research.
2
Epidemiology and Diagnosing Infections
Incidence and prevalence
Epidemiology is the study of factors and mechanisms involved in the
frequency and spread of diseases or health-related problems. The
incidence of a disease is defined as the number of new cases contracted
within a set population in a specific period of time. This kind of
information can provide us with an indication of the spread of a particular
disease. Prevalence is defined as the total number of people infected
within a population at any given time. This means new cases and already
existing cases. Prevalence data can be used to measure how seriously and
for how long a particular disease affects the population.
Morbidity and mortality rates
The morbidity rate of a disease is the number of individuals affected by the disease during a
set period divided by the total population. The mortality rate is the number of deaths due to
a specific disease during a specific period divided by the total population.
When epidemiological studies are used to examine parameters such as particular geographic
areas and degree of harm caused by a disease, they classify diseases as sporadic, endemic,
epidemic and pandemic. Sporadic diseases occur in a random and unpredictable manner and
pose no threat to public health. Endemic refers to diseases that are constantly in the
population but in numbers too low to be a public health problem.
An epidemic occurs when the incidence of a disease suddenly becomes higher than the
normal expected number of individuals affected by the disease. An epidemic causes
morbidity and mortality rates to increase signaling a public health problem. There are several
factors that contribute to the development of an epidemic. For example, the large
populations that inhabit major cities allow the rapid spread of disease within a population.
Increased aging of a population leads to a higher percentage of people who are
immunologically or otherwise compromised, making them more susceptible to infection.
Lack of herd immunity as a result of incomplete or inadequate vaccination is another factor
in epidemics. Perhaps the most important factor is the ability to travel easily and transmit
diseases around the world. When a disease occurs in epidemic proportions throughout the
world, it is called a pandemic.
Epidemics are affected by the type of pathogen causing the disease and
mode of transmission. There are two types of epidemic:
common-source outbreaks and propagated epidemics.
1. Common-source outbreaks:
An epidemic that arises from contact with contaminated substances and
most commonly occurs when a water supply is contaminated with fecal
material or when food is improperly prepared or contaminated in
1
, processing. Although common-source outbreaks affect large numbers of people, they quickly
subside when the source has been resolved.
2. Propagated epidemic:
Results from amplification of the number of infected individuals as person-to-person contact
occurs. Propagated epidemics stay in the population for long periods and are more difficult
to deal with than common-source outbreaks.
Herd immunity
Herd immunity can be conferred through vaccination or can be developed after infection. For
example, in years past smallpox was responsible for millions of deaths. When a vaccination
against the disease was developed and given widely, the immunity of the population as a
whole to this infection increased. It does not mean that every individual in the population is
immune, but individual protection is conferred by limiting the number of possible hosts.
When the population, the herd, had become immune, the disease essentially disappeared
because there were insufficient susceptible hosts to maintain transmission of the pathogen.
Types of epidemiological study
Descriptive epidemiological studies are concerned with the physical aspects of patients and
the spread of disease. These studies include data on the number of cases and on which
segment of a population was affected. They also include location and time of infection, as
well as age, gender, race, marital status and occupation of those infected. Examination of this
type of information can allow epidemiologists to trace the outbreak of disease and possibly
identify the index case, which is the first person to have been infected.
In analytical epidemiological studies the focus is on establishing a cause-and-effect
relationship. These studies are done in conjunction with a control group, which is a
population known to be free of the disease. A retrospective analytical study is one in which
the records of patients who have already contracted the disease are studies. Using this type
of information, workers can take into account a wide variety of factors that preceded an
epidemic so that they can narrow down the potential cause. In prospective analytical studies
data are analyzed as they are collected. Prospective studies consider factors that occur as the
epidemic proceeds. Retrospective studies allow epidemiologists to develop and refine
models to get more accurate predictions for the future, which then allow taking preventative
or limiting measures for the benefit of everyone.
Health departments at the local and state level require that doctors and hospitals report
certain diseases. In addition, some diseases are classified as nationally notifiable, the ones
that are important to control and contain, which means they must be reported to national
centers such as the CDC, the clearing house for epidemiological research.
2
