376 PATHOGENIC MICROORGANISMS 



Some diseases will of necessity remain sporadic because the infec- 

 tion of a new individual can be brought about only by unusually 

 depressed resistance or by accidentally enhanced virulence on the 

 part of the causative agent. And in some diseases transmission 

 requires conditions of contact which are not an ordinary feature 

 of intercourse between the members of communities. Diseases, thus, 

 which under ordinary conditions of civilized life may occur more 

 or less frequently as sporadic scattered cases, may become epidemic 

 only when life in army cantonments, in crowded and unsanitary 

 city quarters, subject to poverty, filth and neglect, produces com- 

 munity susceptibility and facilitates transmission. Such, as we shall 

 see, is the case with many respiratory infections, especially the 

 pneumonias. 



Other diseases, on the other hand, are characteristically epidemic 

 because the ordinary virulence of the microorganisms is such that 

 practically all normal human beings may be regarded as susceptible 

 and because the most important avenues of invasion are open in the 

 course of normal community association. Such diseases are plague, 

 smallpox, cholera, influenza and, to a less extreme degree, the enteric 

 fevers. 



For this reason, the ultimate basis of sanitation in infectious 

 diseases depends upon close observation of the possible sources of 

 infection so that they may be circumscribed before broadcast dis- 

 semination has taken place; it involves the routine safeguarding 

 of ordinary community life in such a way that the avenues of trans- 

 mission for the various possible invaders may be controlled, and 

 finally it necessitates attention to the maintenance of the resistance 

 of the community as a whole, both by the hygiene of every day life, 

 the prevention of undue lowering of resistance of large groups by 

 economic or other hardships, or, as in smallpox and typhoid fever, 

 by the artificial reenforcement of community resistance by methods 

 of immunization. 



The source of infection lies invariably in direct or indirect trans- 

 mission of microorganisms from a human or animal source. Every 

 case of infection represents the possible origin of many others, and 

 necessitates surrounding the patient with all the safeguards appro- 

 priate to the type of infection from which he is suffering, based 

 on knowledge of the manner in which the particular disease is 

 transmitted. If the diseased individual were the only problem, the 

 task would be relatively easy. As we shall see, however, in the 



