GENERAL CONSIDERATION OF FILTRABLE VIRUS 893 



These researches, however, are by no means convincing, and 

 Ewing, 10 while admitting that the vaccine bodies are probably 

 specific for variola, calls attention to the fact that specific 

 cell-degenerations or inclusions are found in diphtheria, measles, 

 glanders, rabies, and other infectious processes, which can not be 

 regarded as in any way related to these diseases etiologically, and 

 suggests the probability of a similar interpretation for the vaccine 

 bodies. Much has been said on both sides of the question since 

 that time, and the problem can not be regarded as settled. The 

 burden of proof, of course, rests upon those who claim the discovery 

 of a specific microorganism, and absolute proof will probably be 

 lacking until our experimental methods are such as will permit of 

 other than purely morphological demonstration. 



Whatever the actual causative agent may be, it is certain that 

 the disease is transmitted with extreme ease actual contact, direct 

 or indirect, with a patient being unnecessary for its transmission. 

 While we have no certain knowledge of the portal of entry through 

 which the virus invades the human body, many considerations have 

 made it seem plausible that this may take place through the mucosa 

 of the upper respiratory tract. 



Our knowledge of the means of defense against the malady is 

 fortunately more advanced than is that of its etiology. It has been 

 known for centuries that one attack of smallpox protects against 

 subsequent attacks. This knowledge was made use of by the physi- 

 cians of ancient China and India, who, during mild epidemics, 

 exposed healthy children to infection, hoping that mild attacks 

 would result which would confer immunity. While dangerous in 

 the extreme, such "variolation," nevertheless, was not without some 

 benefit and was even introduced into Europe in the eighteenth cen- 

 tury by Lady Mary Wortley Montagu. 



Such practices, however, were made unnecessary by the classical 

 investigations of Jenner published in 1798. Jenner, as a student, 

 had been impressed with the fact that country-people who had been 

 infected with a disease known as cowpox, were usually immune 

 against smallpox. His studies and observations came to a practical 

 issue when, in 1796, he inoculated a boy, James Phipps, with pus 

 from a cowpox lesion on the hand of an infected dairy-maid. Two 

 months later the same boy was inoculated with material from a 



10 Ewing, Jour. Mecl. Res., xiii, 1905. 



