PARASITIC BACTERIA. 159 



curs to check their growth. After the individual 

 develops the renewed powers of checking their 

 growth, recovery takes place, and the individual 

 is then, because of these renewed powers of re- 

 sistance, immune from a second attack of the dis- 

 ease for a variable length of time. 



This, in the merest outline, represents the rela- 

 tion of bacterial parasites to the human body. 

 But while this is a fair general expression of the 

 matter, it must be recognised that different dis- 

 eases differ much in their relations, and no general 

 outline will apply to all. They differ in their 

 method of attack and in the point of attack. Not 

 only do they produce different kinds of poisons 

 giving rise to different symptoms of poisoning ; 

 not only do they produce different results in dif- 

 ferent animals ; not only do the different patho- 

 genic species differ much in their power to de- 

 velop serious disease, but the different species are 

 very particular as to what species of animal they 

 attack. Some of them can live as parasites in 

 man alone; some can-live as parasites upon man 

 and the mouse and a few other animals; some 

 can live in various animals but not in man ; some 

 appear to be able to live in the field mouse, but 

 not in the common mouse ; some live in the horse; 

 some in birds, but not in warm-blooded mammals ; 

 while others, again, can live almost equally well 

 in the tissues of a long list of animals. Those 

 which can live as parasites upon man are, of 

 course, especially related to human disease, and 

 are of particular interest to the physician, while 

 those which live in animals are in a similar way 

 of interest to veterinarians. 



Thus we see that parasitic bacteria show the 

 widest variations. They differ in point of attack, 

 ii 



