688 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



Melanesians are very susceptible to measles alid the Malaysians to beri- 

 beri, while other races are relatively immune. 



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 

 undoubtedly 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 apd the association with other diseased 

 persons. The so-called tuberculous diathesis does exist and perhaps we 

 have an explanation of it in anaphylactic phenomena as mentioned 

 previously. Measles and scarlet fever also in certain instances seem to 

 runjn families. 



Individual Imtriunity and Susceptibility. — Variation among indi- 

 viduals 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 

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

 breed and be fed and handled the same as the rest of the herd, still they 

 never become infected. Again, in the human race, with the acute exan- 

 thema tons diseases such as scarlet fever and measles, there are children, 

 fflr 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 the 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, sleep, work and general hygienic conditions. 



Factors of Natural Immunity. — ^The natural immunity of any 

 individual to an infection may be dependent upon several things as 

 follows: 



The Protection 4- forded the Body by the Surfaces. — The body surfaces 

 may be conveniently divided into those which are external and those 

 which are internal. 



Skin and Cutaneous Orifices. — The first protective mechanism that 

 we wish to call attention to is the skin. It is a well-known fact that 

 virulent bacteria are frequently present on the skin of seemingly normal 

 and healthy individuals. Perhaps the most common of these is the 

 Strept. pyogenes and the M. pyogenes vars. aureus and albus. These 

 microorganisms and others live largely as saprophytes, feeding upon 



