310 STRUCTURE AND LIFE HISTORIES 
on the march; yet we could sleep in our damp garments 
without fear of taking cold." 1 
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- 
1 From a letter to the author from John W. Goodsell, M. D., Surgeon 
of the Peary arctic expedition of igo8-og. 
