CHAPTER III. 

 IMMUNITY AND SUSCEPTIBILITY. 



ONE of the most interesting things observed in physi- 

 ology and pathology is the resistance which certain ani- 

 mals show to the invasion of their bodies by the germs 

 of disease. 



Thus, man suffers from typhoid fever, cholera, and 

 other infectious diseases which are never observed in the 

 domestic animals; cattle are subject to a pleuro-pneumo- 

 nia which does not affect their attendants; man, the cow, 

 and the guinea-pig are peculiarly susceptible to tubercu- 

 losis, which the cat, dog, and horse resist; yellow fever 

 is a highly contagious, infectious disease which is almost 

 certain to attack all new arrivals of the human species 

 when epidemic, but which rarely, if ever, attacks animals. 



The popular mind accepts the statement of such facts 

 as these without any other explanation than that the 

 animals are different, and so of course their diseases are 

 different; but the more the scientific man contemplates 

 them, the more complicated the matter becomes; for, 

 while it might be admitted that a difference -in the body- 

 temperature and chemistry might explain why a frog 

 will resist anthrax, which readily kills a white mouse, it 

 will not explain why a house-mouse, whose chemistry 

 must be almost identical with that of the white mouse, 

 can successfully combat the disease. Nor is this all. 

 That one attack of yellow fever, of typhoid fever, or 

 of scarlet fever renders a second attack almost impos- 

 sible is not the less interesting because of its every-day 

 observation. The mouse that has recovered from teta- 

 nus will not take tetanus again, and most interesting and 



