IMMUNITY AND SUSCEPTIBILITY. 543 



cases are on record to show that disease seems to run in families and in 

 localities. For example, tuberculosis and cancer are frequently said to be 

 subject to inheritance or to predisposition in certain cases. It can be easily 

 seen that if one parent is diseased that the germ cell of that parent may 

 be less healthy and when combined with a normal healthy germ cell of the 

 other parent will not give rise to as healthy an individual as when both 

 cells are from healthy individuals. Again, the result when the germ 

 cells of both parents are unhealthy due to the parents being unhealthy 

 is evident. Predisposition seems to resolve itself into the inheritance of 

 a weakened constitution, a constitution which will not withstand the 

 ordinary infections easily. It may be a predisposition to some particular 

 disease or a predisposition to all diseases, infectious and metabolic. 

 Diseases such as tuberculosis are so prevalent that it is very possible that 

 infection may take place and it be interpreted as inherited because the 

 parent died of the same thing. As mentioned above, it may be that 

 the true explanation of the phenomena of predisposition is found in 

 anaphylaxis or the sensitization to various proteins of microorganisms. 

 Further work is necessary along these lines. 



.* I IMMUNITY. 



Immunity and susceptibility to disease are always relative and never 

 absolute; that is, it is always possible to produce some sort of an infection 

 in a supposedly immune animal by modifying the conditions under which 

 the animal is accustomed to live. For example, the chicken is immune 

 to tetanus but by keeping this animal for some time at a temperature 

 higher than its normal it may be infected. The cow cannot ordinarily 

 be infected with typhoid but when large numbers of the B. typhosus 

 are injected under the skin an abscess may be produced. These and 

 many other examples might be mentioned. Our standard of immunity in 

 a particular animal is based upon the conditions as they exist naturally 

 and on the average resistance of animals of the same species. 



Immunity to disease may be of two kinds, natural and acquired. 

 Natural immunity is that resistance which is possessed normally by an 

 individual. Acquired immunity is that resistance which is acquired by 

 having an infection or by being vaccinated or immunized against it with 

 the specific etiological microorganism or its antiserum. 



NATURAL IMMUNITY AND SUSCEPTIBILITY. At this time attention is 

 directed to certain forms of natural immunity and susceptibility. 



