(1) Who has the disease? In other words, which populations, or groups of
persons within a given population, have high or low rates of a disease?
(2) Where is the disease? More specifically, in what regions of the city, the
country, or the world is the disease most prevalent and most highly concentrated?
(3) How is the disease transmitted? Is it spread by personal contact, by
means of an arthropod vector, via food and water, or by some other means?
(4) When does the disease occur? Is it endemic, existing at fairly constant
levels? Is it epidemic, flaring up at certain times of the year? What is the incubation
time? What is the normal course of the disease in terms of its duration?
b. An example of the epidemiology of a disease is that of syphilis. If we were to
examine a textbook of medicine and look up the epidemiology of syphilis, we should
very likely find a description similar to the following.
(1) First, who gets syphilis? We would find that the disease is most
prevalent in the younger, most sexually active group of adults--chiefly in the 20-25 year
age group. It is not as common among teenagers as is commonly believed. Syphilis is
much more commonly reported by males than by females.
(2) Second, where do we find the disease? We learn that syphilis is a
disease of large cities. It has a much lower incidence in rural areas. High rates are
found in the United States, the Scandinavian countries, Western Europe, and Japan.
(3) Third, how is syphilis transmitted? Primarily by sexual intercourse,
although congenital syphilis may occur occasionally in infants born of syphilitic mothers.
In rare instances, the disease may be contracted by kissing. Cases acquired by intra-
rectal intercourse and oral-genital contacts are not infrequent.
(4) Fourth, when does syphilis manifest its symptoms? We find that
syphilis, compared to most infectious diseases, has a fairly long incubation period.
Characteristically, the first symptoms appear in about three weeks, followed (in the
untreated state) by secondary manifestations after about four to six weeks. These
manifestations disappear spontaneously, even in the absence of treatment; however,
they may be followed--after a period of from 5 to 20 years of latency--by explosively
destructive lesions of late syphilis attacking the skin, bone, mucosal surfaces, or central
c. From the information obtained in a situation such as that illustrated above, we
are able to plan a preventive medicine effort toward eradicating (or at least minimizing
the effects of) a disease. We know which groups we are dealing with, where the
problem occurs, the means by which the disease is spread (and therefore the means by
which to prevent its spread), and the critical periods of time during which we must act to
obtain optimum benefit from our efforts.