544 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



Racial Immunity and Susceptibility. It is a familiar fact that certain 

 species of animals and certain races of man differ in their resistance and 

 their susceptibility to infectious diseases. As examples of racial immu- 

 nity among animals may be mentioned the native cattle of Austria-Hungary 

 and of Japan which are relatively immune to bovine tuberculosis, a 

 disease which causes great loss among other races. Again, the sheep 

 of Algeria are relatively immune to anthrax while all other sheep are 

 extremely susceptible. Field mice are immune to glanders while the 

 common house mouse is susceptible. The negro is more resistant to 

 the infectious microbic agent of yellow fever than other races, but is 

 without doubt more susceptible to tuberculosis. The Melanesians 

 are very susceptible to measles and the Malaysians to beri-beri. Other 

 races are relatively immune. The Japanese are said to be more resist- 

 ant to scarlatina than other races. 



Familial Immunity and Susceptibility. It is true that certain families 

 vary in their immunity and susceptibility when compared with other 

 families in the same community. For example, tuberculosis undoubt- 

 edly shows a tendency to run in families. In determining a case of this 

 kind it is, of course, necessary to take cognizance of the environment 

 of the individual and the association with other diseased persons. The 

 so-called tuberculous diathesis does exist and perhaps we have an ex- 

 planation of it in anaphylactic phenomena as mentioned previously. 

 Measles and scarlet fever also in certain instances seem to run in families. 



Individual Immunity and Susceptibility. The same variation among 

 individuals associated together is noted in regard to their resistance and 

 susceptibility to disease. It is well known, for example, that in a herd of 

 cattle, which are in the main tuberculous, there are certain individuals 

 which never contract the disease. These animals may be of the same 

 breed and be fed and handled in the same manner as the rest of the 

 herd, still they never become infected. Again, in the human race, with 

 the acute exanthematous diseases such as scarlet fever and measles, there 

 are children, for example in the same family and of nearly the same age 

 and living under exactly the same conditions, who contract the disease 

 and others who do not. The exact cause of individual, familial and 

 racial immunity cannot be satisfactorily explained at the present time. 

 There is also a variation in the individual's resistance at different times 

 dependent upon food and hygienic conditions. 



FACTORS OF NATURAL IMMUNITY. The natural immunity of any 



