to a water pump. He concluded that individuals who died from cholera drank
water contaminated with human feces. Snow’s study and his method of analyz-
ing where and when a disease could occur and the transmission of that disease
within a given population gave way to a new approach in medical research and
epidemiological studies.
It was not until 1883 that the cholera bacterium Vibrio choleraewas identi-
fied by Robert Koch. After the studies of Snow, other investigators conducted
epidemiological studies. Epidemiologists now use three types of studies when
determining the occurrence of disease: descriptive, analytical, and experimental.
Descriptive epidemiologyis the collection of all data described in the occur-
rence of disease. This data includes the number of cases, which portion of the
population was infected, where the cases occurred, and information about
the affected individuals (race, sex, age, occupation, marital status, and socio-
economic status).
Analytical epidemiologyanalyzes the cause of a disease and the effect of the
disease in a given population. Here epidemiologists compare a group of people
who have the disease with people who do not have the disease.
Experimental epidemiologyare studies designed to test hypotheses for the
source of a particular disease. Experiments are conducted on human or animal
subjects to test the hypothesis. An example is the testing of an experimental drug
designed to control a specific disease. A group of infected individuals is divided
randomly so that one group receives the drug and the other receives a placebo.
Aplacebois a substance that has no effect on the individual receiving it, but the
individual believes he or she is receiving treatment. If those people who were
treated with the drug recover faster than those taking the placebo, investigators
can conclude that the drug treatment is effective.
Control of Communicable Diseases
There are different ways of controlling or limiting the spread of communicable
diseases. These methods include isolation, quarantine, immunization, and vec-
tor control.
Isolationrequires that a patient infected with a communicable disease be pre-
vented from making contact with the general public. The Centers for Disease
Control and Prevention has designated five categories of isolation: strict, pro-
tective, respiratory, enteric, and wound and skin.
Quarantinerequires the separation of animals and humans that have been
infected or exposed to a communicable disease from the general public.
CHAPTER 13 Epidemiology and Disease^205