ulation. Epidemiologists use this information to design ways of preventing and
controlling outbreaks of disease.
Epidemiology is considered a branch of microbiology because microorgan-
isms cause many diseases. It can also be considered a branch of ecology because
of the relationship among pathogens, their hosts, and the environment. The sci-
ence of epidemiology provides the methods and information that are used to
understand and control outbreaks of diseases in human populations, making it
important for public health. An epidemiologist is a person who is trained to iden-
tify and prevent diseases in a given population or a medical doctor who is trained
to identify and treat diseases in individual people.
Epidemiologists are concerned with the frequency or prevalence of diseases
in a given population. An epidemiologist will identify the factors that cause dis-
ease or how that disease is transmitted and how the spread of communicable and
noncommunicable diseases can be prevented. The incidence rateof a disease is
the total number of new cases seen within a calendar year. The prevalence of a
disease is the number of people infected at any given time. The prevalence rate
is the total number of old and new cases of a disease. Frequencies are also
expressed as proportions of the total population. The morbidity rateis the state
of illness or the number of people in a given population that are ill. This is
expressed as the number of cases per 100,000 people per year. The mortality rate
is the number of people that are dead. This is measured as the number of deaths
from a specific cause per 100,000 people per year.Classification of Disease
Epidemiologists measure the frequency of diseases within a given population in
regards to the geographical size of the area and the amount of damage the dis-
ease inflicts on the population. Diseases can be classified as endemic, sporadic,
epidemic, or pandemic.
An endemic diseaseis the average or normal number of cases of a disease in
a certain population.The number of people contracting the disease and the sever-
ity of the disease is so low that it raises little concern and does not constitute a
health problem. An example is the varicella-zoster virus (the virus that causes
chickenpox). Chickenpox is an endemic disease that usually affects children and
is seasonal. An endemic disease can give rise to epidemics.
Asporadic diseaseoccurs when there are small numbers of isolated cases
reported. Sporadic diseases do not threaten the population.
An epidemic diseasearises when the level of disease in a certain population
exceeds the endemic level. This disease will cause an increase in mortality rate(^198) CHAPTER 13 Epidemiology and Disease