io8 Immunity 



against scarlatina, and that negroes are immune against 

 yellow fever, but increasing information is to the contrary c 



Human beings suffer from typhoid, cholera, measles, 

 scarlatina, yellow fever, varicella, and numerous other 

 diseases unknown among the lower animals, even those 

 domestic animals with which they come in close contact. 

 They also suffer from Malta fever, 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 juices. 



These facts are 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 products 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 necessarily, 

 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. 

 The reactions in general correspond with those of natural 

 immunity, and comprise mechanisms for overcoming the 



