476 PATHOGENIC MICROORGANISMS 



human beings is comparatively high. The disease will not occur in an 

 individual simply because he has received the virulent organisms by 

 the proper route, from a case or a carrier, but, in addition to this, 

 there must be coincident hygienic defects which temporarily depress 

 his resistance. A temporary coincidence of two factors, therefore, 

 transmission of the organisms and increased susceptibility, must occur, 

 and it is plain that in such diseases epidemic spread cannot take place 

 to any extensive degree unless both of these factors, widespread trans- 

 mission and depression of resistance, become generalized. In such 

 cases, therefore, while proper safeguards against dissemination of the 

 organisms must be developed, yet the efforts of the sanitarian should 

 focus particularly upon measures by which the normal resistance of 

 the community is maintained. 



As a matter fact, pneumonia epidemics do not occur as a rule in 

 well nourished and housed communities. The epidemic form of the 

 primary disease develops only under such conditions as those prevail- 

 ing in army camps during the cold weather, when men are crowded 

 together in sleeping quarters, and are developing colds and coughs, 

 and are, at the same time, exposed to unusual conditions of life, cold, 

 wet, unaccustomed food and hard work. Exceptions to this are, of 

 course, epidemics like those that have occurred in Panama and South 

 Africa, but in these cases the community in which the disease was 

 prevalent consisted very largely of tropical negroes, whose greater 

 susceptibility to pneumococcus infection is well known. 



In a number of epidemics in which we have had the opportunity 

 of studying cases, it was quite apparent that the susceptibility factor 

 was the determinative one in individual instances. Surveys showed 

 that, while it frequently happened that a number of cases came from 

 the same tent, the infections were often of different bacterial types. 

 While direct transmission from one case to another often seemed to 

 be circumstantially proved, in only a few instances at a certain camp 

 in which these studies were made, were such cases associated with a 

 single type. On the other hand, a certain regiment which was ordered 

 to the shooting range during a very wet spell, marching in the rain 

 and camping on wet ground, developed 26 pneumonias within 16 days. 

 Analysis showed that these cases were caused by all four pneumococcus 

 types without particular relationship between contacts and types. On 

 the other hand, a considerable number of men were found, in this 

 same regiment, at that time, to be carriers of virulent pneumococci 

 and streptococci without coming down with the disease. 



