ASPECTS OF THE EPIDEMIOLOGY OF TUBERCULOSIS? 
By LELAND W. PARR 
The George Washington University 
Despite the difficulty the American Public Health Association had a 
short time ago in settling upon a definition of an epidemiologist, I 
believe it is not impossible to say what epidemiology is. Epidemiology 
is the ecology of disease. It is the life history and environmental re- 
lationships of disease. It places less emphasis on how disease acts on 
the individual and more on its mass manifestations; little on symptoms, 
much on how it spreads and is influenced by all possible variant factors. 
The study of tuberculosis is tremendously complex, and the results 
that have been obtained are confusing. This is not because the organ- 
ism causing the disease is difficult to obtain and study. True, Mycobac- 
terium tuberculosis grows slowly, but we have long had satisfactory 
culture mediums and suitable experimental animals are readily avail- 
able. There is, however, no disease concerning which there are more 
disputed concepts and theories. Shortly after the tubercle bacillus 
invades the body successfully the tissues take on a new and specific 
capacity to react. If into the skin of such a person a tiny bit of the 
soluble protein of the tubercle bacillus is injected, there is a decisive 
response. The area becomes inflamed, slightly raised, unusually firm, 
and somewhat painful. It is, in fact, a typical area of response in in- 
flammation. This reaction reaches its height on the second and third 
day and thereafter slowly fades away. This is a positive tuberculin 
test. By contrast, a person who has not been successfully invaded by 
the tubercle bacillus will give no reaction to a similar injection or in- 
deed to one many times stronger in its tuberculin content. 
The condition of the individual that causes him to react to the injec- 
tion of tuberculin is the “tuberculin type of hypersensitivity.” It 
would seem simple to determine whether it is better to be tuberculin 
positive or tuberculin negative, but it is not. Is this tuberculin type 
of hypersensitivity the same thing as immunity? It is not easy to 
decide, and any answer given will be disputed. Woodruff and Kelly 
(1942) observed: “Before tuberculosis can be controlled successfully 
1 Address of the retiring president of the Washington Academy of Sciences delivered at 
the 324th meeting of the Academy on February 17, 1944. Reprinted by permission from 
the Journal of the Washington Academy of Sciences, vol. 34, No. 6, June 15, 1944. 
477 
