378 PATHOGENIC MICROORGANISMS 



safeguard a community, it is necessary to know where in the body 

 of the sick or the carrier the organisms are to be found; how they 

 may leave the body; what the viability of the infectious agent is in 

 nature; how long it can live under different conditions of environ- 

 ment in the interval between its leaving one body and entering 

 the next, and by which channels it most easily infects. Also, the 

 habits of insects that can carry disease must be studied. 



To interrupt the chain of transmission from source to victim, 

 .also, there must be a routine organization for the safeguarding of 

 water, food and other agencies which in crowded communities are 

 always in danger of contamination. And in special cases it may 

 involve emergency measures of personal hygiene, engineering prob- 

 lems and insect destruction. 



In thinking of infectious diseases from the point of view of 

 sanitation it is well to carry them in one's mind in the tentative 

 classification based upon means of transmission, since for each par- 

 ticular subdivision of this kind, preventive measures will fall into 

 a definite common plan of procedure. 



Thought of in this way, all the infectious diseases fall into four 

 groups : 



(1) The first of these consists of those transmitted by the respira- 

 tory channels. This means that the infectious virus leaves the case, 

 convalescent or carrier, witli the saliva or mucus of the upper 

 respiratory passages and enters the new victim by the same chan- 

 nels. On this basis, the respiratory group would consist in fhe 

 common cold, the pneumonias, influenza, scarlet fever, measles, 

 smallpox, chickeiipox, mumps, whooping cough, tuberculosis, diph- 

 theria, meningitis, poliomyletis and a few others. 



In dealing with such diseases, the task is to suppress conditions 

 which would make it possible for a wholesale distribution of sputum, 

 for close contact between individuals in sleeping quarters, the 

 avoidance of crowding in homes, schools, institutions, etc., ventila- 

 tion, dust prevention, and, in short, to diminish as much as possible 

 the opportunities for close individual contact in closed spaces. In 

 this group of diseases, prevention is most difficult because it is quite 

 obvious that contacts need no1 be of long duration, and that close 

 association in public vehicles, and in the ordinary course of business 

 and social intercourse may suffice for transmission. In these dis- 

 eases, particularly, when there is universal susceptibility, as in the 

 case of influenza or smallpox, or some of the exanthemata in the 



