Lesson 1: Introduction to Disease Transmission and Epidemiology

1-12

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1-12. EPIDEMIOLOGY

a. Epidemiology is the study of the distribution and determinants of disease in human populations. It is the basic science of preventive medicine. Epidemiology is to preventive medicine what mathematics is to chemistry or physics; it is the practical tool used to study disease problems in the community. The distribution and determinants of a disease can be expressed by answering four questions.

(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 nervous system.

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.

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