CHAPTER XLII. 



PROTOZOAN-LIKE BODIES IN SMALLPOX AND ALLIED DISEASES 

 (COWPOX, HORSEPOX, SHEEPPOX) AND IN SCARLET FEVER. 



THE question as to the chief exciting factor in smallpox and allied 

 diseases, according to some authorities, is still undecided, while accord- 

 ing to others it is settled beyond doubt. These latter investigators con- 

 sider that certain bodies found chiefly in the epithelial cells of the skin 

 and mucous membranes in the specific lesions of these diseases are 

 protozoa causing the diseases. 



The different diseases named in the chapter heading, excepting only 

 scarlet fever, if not identical, are closely allied. Indeed, the following 

 facts seem to prove that at least cowpox and variola are very closely 

 related diseases, if not the same disease : First, smallpox virus inocu- 

 lated into calves produces after passage through several animals an 

 affection exactly similar to cowpox. The successful inoculation of the 

 first series of cattle from smallpox is a matter of great difficulty, but so 

 many experimenters have asserted that this has been done that there 

 seems to be no doubt as to its truth. In our laboratory not one of many 

 attempts to accomplish it has been successful. Second, both when occur- 

 ring in nature and when produced by experiment the lesions of the two 

 diseases are similar. Third, monkeys have been successfully protected 

 against either disease by previous inoculation of the other; also, obser- 

 vations go to show that human beings inoculated with cowpox vaccine 

 are not susceptible to inoculation with smallpox virus, and that those 

 who have within a varied time passed through an attack of smallpox 

 cannot be inoculated successfully with cowpox vaccine. These facts 

 seem positively to prove that the two diseases are produced by organ- 

 isms originally identical, one being modified by its transmission through 

 cattle, the other through human beings. 



The immunity caused by successful vaccination is not permanent, and 

 varies in its duration in different individuals. Although it may give some 

 protection from smallpox for ten or fifteen years, it is not well to count 

 on immunity for more than two years, and whenever one is liable to 

 exposure it is well to be vaccinated. If this vaccination were unnecessary 

 it will not be successful, while if it is successful we have reason to 

 believe the individual was open to at least a mild smallpox infection. 



Protective Substances Present in the Serum of Animals after Successful 

 Vaccination. It has been frequently shown that the blood serum of a 

 calf for several weeks after successful vaccination possesses feeble 

 protective properties, so that the injection of one to two litres of it into 

 a susceptible calf would prevent a successful vaccination. A further 



