174 INFECTIOUS AND PARASITIC DISEASES. 



and parasites in both air and foods, and in the case of 

 many diseases both methods of conveyance are possible. 

 Yet because some diseases are nearly always, or even 

 exclusively, contracted in one of these v^ays, it has 

 seemed best to make a division of diseases into those 

 inspired, and those taken in with foods and water. 

 For example, neither Asiatic cholera nor dysentery are 

 ever air-borne; nor is pneumonia contracted in either 

 food or drink; yet in diphtheria, scarlet fever, small-pox, 

 etc., both air and foods are fruitful sources of infection. 

 The air, however, as a source of contagion, has been 

 much exaggerated in the past, particularly where trans- 

 mission of a disease over great distances has been laid 

 at its door. With the exception of diseases conveyed 

 by insects, such as mosquitoes, which may be blown 

 by winds, there is really no readily communicable 

 disease which is dangerous to others in the open air 

 at a few yards distance. The fear that many persons 

 have of passing a house where a contagious disease 

 exists is without foundation. Carelessness within the 

 house may be responsible for every room being danger- 

 ous — but that does not affect the air outside. When 

 carefully investigated the so-called air-borne infections 

 are discovered to have been carried by a third 

 person, or by some article used and soiled in the sick- 

 room. Usually it is the former. The truth is graphic- 

 ally presented by Dr. Rosenau who quotes Dr. 

 J. H. White, ''that infectious diseases are more often 



