362 Human Physiology. 



diseases, or have them so lightly that they are scarcely percep- 

 tible, we know that some predisposition must unite with the 

 determining cause. As most persons have these diseases but. 

 once, it would seem that some element of predisposition is r 

 by the action of the disease itself, eliminated from the system. 

 It would appear also that one who has had any of these dis- 

 eases is less liable to have the others ; that they purify the 

 system so as to make it less susceptible to disease in general. 



We are sometimes told that small-pox and cholera can be 

 generated at any time, and anywhere, by the conditions of filth 

 that favour their spread as an epidemic, but there is no good 

 ground for this assertion. Most of the great towns of Europe 

 and America had been in bad sanitary conditions for a long 

 period, but the Asiatic cholera never was known among them 

 until it was brought from Asia. When small-pox broke out as 

 an epidemic in so many English and American towns in 1871-2, 

 these towns were in no worse sanitary condition than they had 

 been for twenty years, and many of them had greatly improved;, 

 nor had vaccination been more neglected. 



In the enthetic or implanted diseases, and especially the 

 most widely spread, and, as many think, the most virulent 

 and mischievous of them, syphilis, we have similar conditions 

 a constitutional liability and a specific poison. Some persons 

 do not take syphilis, and cannot even be inoculated with its virus. 

 In some it is a comparatively mild disease ; in others it pro- 

 duces the most terrible ravages. Some thrgw it off readily ; 

 others can never get free from it, and transmit it in various 

 modified forms to their posterity. But of the real, determining 

 cause of syphilis we know almost nothing. We do not know 

 the time or manner of its origin, or its first 'appearance in 

 Europe. It cannot be shown that it has been caused by 

 excessive or promiscuous intercourse of the sexes : for in 

 periods of the greatest and most wide-spread profligacy, it was,, 

 so far as we can judge, unknown. And now it is never known 



