236 HUMAN DISEASES 



unable to overcome the microbes except after a prolonged struggle 

 from which no acquired immunity results. 



395. It is a noticeable and very important fact that immunity 

 acquired against any acute disease does not confer immunity 

 against any other. Thus experience of smallpox or scarlatina 

 affords no protection against measles and whooping-cough. The 

 toxins differ, and therefore immunity to each disease must be 

 acquired separately. 



396. Obviously, the various species of microbes are adapted 

 by different means to the environment. In the search for nutri- 

 ment they enter the body in different ways. Some species defend 

 themselves from the phagocytes by means of toxins which they 

 secrete into the surrounding medium ; others have evolved great 

 powers of resistance which doubtless depend in part at least on 

 toxins retained within themselves. In no case is the death of the 

 host they inhabit of advantage to the microbes. They are adapted 

 for the temperature, nutriment, and other conditions met in the 

 living body. The dead body becomes the prey of the bacteria of 

 putrefaction. The death of the individual, the cell-community 

 attacked by them, is comparable to the destruction of a hive by 

 the bee-keeper in an attempt to secure the honey. Continued 

 multiplication of the microbes within the system, however, insures 

 the death of the host, if only by interference with the functions of 

 life. The preservation of their species, therefore, demands that 

 the microbes shall pass through an unending succession of living 

 individuals. In acute diseases this is secured by rapid multiplica- 

 tion and migration to fresh and susceptible persons during the 

 short interval of safety afforded by the toxins before acquired 

 immunity or death supervenes. In the more chronic maladies there 

 is less need for rapid multiplication and migration. The host is 

 not poisoned. His continued existence is an advantage as afford- 

 ing a continuous supply of nutriment and prolonged opportunities 

 for infecting other individuals. However quickly the microbes may 

 multiply, they never increase at the tremendously rapid rate which 

 characterizes the organisms of the acute diseases. Thus, in a few 

 days or weeks, measles, influenza, or smallpox may spread over 

 a vast area of country, and infect millions of people. The progress 

 of tuberculosis and leprosy is always much slower. 



