HOST-PARASlTE RELATIONSHIP 51 



resistant recruits remain well unless subjected to conditions that 

 would lower their innate resistance. 



There are several reasons why great caution and conservatism 

 should be used in translating the results of experimental epi- 

 demiology to human field conditions. These reasons may be sum- 

 marized as follows: 



1. There are many species differences between man and 

 mice. While Webster and Hodes have apparently shown that 

 there is no tendency for susceptible mice to become immunized 

 through herd exposure, their experiments do not prove that 

 susceptible humans will react in a like manner. 



2. Man is a heterogeneous mixture as far as breeding is con- 

 cerned while the mouse research has been done with inbred 

 strains of known pure lines of mice whose susceptibility and 

 resistance could be determined. 



3. The experimental epidemics in mice were carried out 

 under controlled conditions as to recruits, diet, temperature, 

 humidity, etc. This is in great contrast to epidemic disease in 

 man. Such epidemics occur spontaneously in a mixed popula- 

 tion that exhibits great variation in age, history of previous 

 disease, diet, health habits, natural fitness, occupation, previ- 

 ous immunization, etc. The source of the infection, the num- 

 ber of susceptibles, resistants, and carriers, as well as the num- 

 ber and condition of new recruits, are usually unknoAvn. 



4. In a mouse population there is no conscious effort on the 

 part of individuals to protect themselves and others against 

 infection such as occurs during epidemics in man. Since there 

 is in every human population great variation in education, 

 knowledge, wisdom, and prejudice, it is not surprising that co- 

 operation of the public in applying what appears to be intelli- 

 gent methods of control is not accomplished. 



In spite of these reasons for caution it would seem that experi- 

 mental epidemiology is making major contributions to our knowl- 

 edge of infection and host-resistance. 



In man the carrier state (both temporary and chronic) presents 

 an interesting hast-parasite relationship that in many cases devel- 

 ops into a public health problem. There is a great deal of data 

 bearing upon the incidence of typhoid, diphtheria, meningococcus 

 and pneumococcus carriers in a population but relatively little 



