488 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
One other point that Frost makes deserves our attention. He states: 
“It is highly probable that the cyclic changes in prevalence which 
are observed in some diseases are brought about chiefly by evolutionary 
changes in the characteristics of the specific microorganisms, the causes 
of which are to be found in uncontrolled natural forces.” Frost men- 
tions scarlet fever and diphtheria as two of the diseases that within 
the past 100 years have greatly changed, although in the case of 
diphtheria the change reversed itself and diphtheria is again a prob- 
lem of some significance in parts of the world. Smallpox, since the 
Spanish American War, has been relatively mild when it has occurred 
in this country, and in 1942 caused but two deaths. It could be pos- 
sible that cyclic changes may be taking place in the nature of the 
tubercle bacillus making it less invasive, but whether this is so, how 
long it will continue, or whether it will reverse are propositions very 
difficult of proof. Again, case finding among medical students and 
physicians yields results with suggestive implications for this point. 
Case finding means looking for cases of a given disease. It is done 
to discover unrecognized cases that should be brought under treatment 
for their own good and isolated or educated so that the public health 
may be protected by removing active sources of infection. Although 
useful for several diseases such as malaria and hookworm, even syphilis, 
case finding is particularly adapted to tuberculosis. It is possible 
through tuberculin testing to discover those belonging to the tuber- 
culin-positive group of persons who can have tuberculosis and, by 
X-ray examination, to detect which of these have physical signs almost 
certain before long to produce clinical symptoms. Such individuals 
may be satisfactorily arrested with a minimum of treatment and loss 
of time whereas if the minimal case is not discovered in its incipiency 
a moderately advanced or even a far advanced case may result which 
is difficult or impossible to arrest. The great advantage to the careful 
examination of the would-be soldier or sailor is that tuberculosis is 
discovered, as never before, in the stage in which it is possible to do 
something about the matter. From the first approximately 400,000 
men appearing for the Canadian Army, 1 percent were rejected for 
tuberculosis. Of 3,530 of these rejectees, there were 1,970 with minimal 
tuberculosis, 1,298 moderately advanced cases, and 262 far advanced 
cases. This ratio of the different clinical types (and the same is true 
for all other large-scale screenings) is the exact reverse of what occurs 
when we let nature take its course. In the past, minimal cases have 
been a minority in the treatment program with moderately advanced 
and far advanced cases constituting the great majority of cases coming 
to the attention of the physician and the care of his sanatorium. It 
is to be hoped that although we are at war care will be taken that the 
