3IO STRUCTURE AND LIFE HISTORIES 



on the march; yet we could sleep in our damp garments 

 without fear of taking cold."^ 



292. Immunity. — This is not the place to discuss the 

 various theories of immunity, but the fact should be noted 

 in passing. It is a common belief that one who has had 

 the measles or the mumps cannot have the same disease 

 again. While this is extremely doubtful, it is known that 

 once having a disease does render one less liable to con- 

 tract it a second time. When the germs multiply in the 

 body with the first attack, the toxin they produce stimulates 

 the various cells to secrete an antitoxin, which counteracts 

 the toxin, or poison. Some persons appear to be naturally 

 immune from certain diseases {e.g., hay fever), while others 

 are specially susceptible. 



293. Disease Carriers. — Persons who are immune may, 

 however, unknowingly transmit the disease-germ from 

 one person to another. They are called "carriers." 

 Typhoid fever, caused by Bacillus typhosus, is often trans- 

 mitted by "typhoid carriers," a recent case being that 

 of "Typhoid Mary," a domestic servant near New York 

 City, who for several years endangered the lives of others 

 in homes and hospitals where she was employed. The 

 menace of such persons to public health justifies their 

 permanent isolation. 



294. Combating Disease. — There are two ways of 

 combating disease in either plants or animals: (i) to 

 guard against it in advance (prophylaxis), thereby en- 

 deavoring to prevent its appearance ; (2) to treat it after 

 it has appeared. Obviously the former is the more im- 



' From a letter to the author from John W. Goodsell, M. D., Surgeon 

 of the Pciiry arctic expedition of 1908-09. 



