g6 Immunity 



have more or less resemblance to one another; thus, anthrax is 

 essentially a disease of warm-blooded animals, though certain ex- 

 ceptions are observed, and Metchnikoff has found that hippo- 

 campi (sea-horses), perch, crickets, and certain mussels are 

 susceptible. Among the warm-blooded animals anthrax is most 

 frequent among the herbivora, though some carnivora may also 

 be infected. 



Close relationship is not, however, a guarantee that animals 

 will behave similarly toward mfection. The rabbit, guinea-pig, 

 and the rat are rodents, but though the rabbit and guinea-pig are 

 susceptible to anthrax, the rat is immune. This is still better 

 exemplified in the susceptibiUty of mice to glanders. The field- 

 mouse seems to be the most susceptible of all animals to infection 

 with Bacillus mallei; the liouse mouse is much less susceptible, and 

 the white mouse is immune. Mosquitos, though closely related, 

 are different in their susceptibility to the malarial parasite. Among 

 the members of the human species, it has been asserted that Mon- 

 gohans, and especially Japanese, are immune against scarlatina, and 

 that negroes are immune against yellow fever, but increasing in- 

 formation is to the contrary. 



Human beings sufler 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, sufier 

 from distemper, 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 agamst 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 agamst 

 any form of bacterio-toxin usually, though not necessarily, deter- 

 mines that the micro-organism, though it may be able to invade 

 the body, can do very little harm. 



