DISEASES OF UNKNOWN ETIOLOGY 419 



in other cases suitable methods of staining or demonstration 

 have not yet been developed. 



Several of the diseases discussed here below are exanthe- 

 mata, that is, they are marked by an eruption of the skin. 

 Such, for example, are the poxes (smallpox, chicken pox 

 and the poxes of the various animals, scarlet fever and 

 typhus fever). The diseases which come under the cate- 

 gory of those whose causes are not certainly known are the 

 following: Smallpox, eowpox, sheep pox, chicken pox, 

 horsepox, measles of various kinds, scarlet fever, infantile 

 paralysis, foot and mouth disease, rabies, hog cholera, 

 influenza, swamp fever, typhus fever and trench fever. 



Smallpox 



The disease smallpox in man has been described in 

 medical literature for at least a thousand years. It is also 

 known as variola. It is among the most contagious of 

 human diseases and can be transmitted to some of the lower 

 animals. Particularly characteristic of the disease are 

 the skin eruptions which develop as vesicles and finally 

 contain pus. These, upon healing, develop scars of variable 

 extent. Smallpox has an incubation period of about 12 

 days. 



The disease is of particular interest because it was the 

 first for which a practicable method of vaccination was 

 developed. It was shown by Edward Jenner in 1798 that 

 vaccination by means of material taken from lesions of cow- 

 pox in cattle will result in an immunity in man to smallpox. 

 Naturally the question at once arises, are eowpox and small- 

 pox identical, that is, caused by the same organism. There 

 is good reason to believe that they are. Several investi- 

 gators have found it possible to develop the disease eowpox 

 in calves by inoculation with the virus of smallpox secured 

 from human cases. Apparently this is not readily accom- 

 plished, however, showing that there is some difference in 



