Acquired Immunity 87 



yellow fever, though not all the facts are definitely known 

 regarding these usually cited examples. 



Human beings suffer from typhoid, cholera, measles, 

 scarlatina, yellow fever, Malta fever, and numerous other 

 diseases unknown among the lower animals, even those 

 domestic animals with which they come in close contact. 

 They also suffer from anthrax, rabies, glanders, bubonic 

 plague, and tuberculosis, which are common among the lower 

 animals. Animals in turn suffer from distemper, hog cholera, 

 Texas fever, swine-plague, chicken cholera, mouse septicemia, 

 etc., the respective micro-organisms of which are not known 

 to infect man. 



It has already been pointed out that mongooses and 

 hedgehogs are immune against the venom of serpents from 

 which other animals quickly die. The tobacco worm lives 

 solely upon tobacco-leaves, the juice of which is intensely 

 poisonous to higher animals, and is also a good insecticide. 

 Boxed cigars and baled tobacco are often ruined by the 

 larvae of a small beetle that feeds upon them, and a glance 

 over the poisonous vegetables will show that few of them 

 escape the attacks of insects immune against their active 

 juices. 



This collection of facts is sufficient to show that many 

 animals are by nature immune against the invasion of 

 microparasites of certain kinds, and that they are also at 

 times immune against poisons. Immunity against one kind 

 of infection or intoxication is, however, entirely independent 

 of all other infections and intoxications. Immunity against 

 infection usually guarantees exemption from the toxic prod- 

 ucts of that particular micro-organism, though experiment 

 may show the animal to be susceptible to it. Immunity 

 against any form of bacterio-toxin usually, though not neces- 

 sarily, determines that the micro-organism, though it may be 

 able to invade the body, can do very little harm. 



ACQUIRED IMMUNITY. 



Acquired immunity is resistance against infection or in- 

 toxication possessed by certain animals, of a naturally sus- 

 ceptible kind, in consequence of conditions peculiar to them 

 as individuals. It is a peculiarity of the individual, not of 

 his kind, and signifies a subtile change in physiology by 

 which latent defensive powers are stimulated to action. 



