60 INFECTION AND RESISTANCE 



or not a tendency to tuberculosis can be inherited is still an open 

 question. In most cases it is more than probable that the supposedly 

 inherited tendency to tuberculosis is not really an inherited sus- 

 ceptibility, but rather an actual infection acquired during childhood 

 from the parents. Cornet and Kossel, 13 who have recently sum- 

 marized the statistics dealing with this problem, have come to the 

 conclusion that this factor, namely, infection from the parents, 

 probably is the cause of the greater frequency of tuberculosis among 

 children of tuberculous parents, and that there is no definite proof 

 of inherited susceptibility. 



ACQUIRED IMMUNITY AND IMMUNIZATION 



We have outlined in the preceding pages the differences in sus- 

 ceptibility to various diseases apparent among different species of 

 animals, and have noted that the degree of resistance of some animals 

 to infection with germs rapidly fatal to others is often sufficiently 

 well-marked to be termed "immunity." Such immunity, because it 

 is a natural biological attribute of the species, as much a character- 

 istic property as are its anatomical or physiological properties, has 

 been spoken of as "Natural Immunity." 



It is a matter of common knowledge, however, that among 

 species of animals readily susceptible to certain infections resistance, 

 or even extreme resistance, i. e., immunity, may be acquired by an 

 attack of the disease. Thus human beings who have recovered from 

 plague, small-pox, typhoid fever, cholera, the exanthemata, mumps, 

 typhus, yellow fever, and a number of other conditions do not ordi- 

 narily contract the disease a second time. In some of these condi- 

 tions, notably cholera, plague, typhoid fever, and small-pox, the rule 

 is almost invariable. In others, such as measles, scarlet fever, and 

 mumps, a second attack may occur, though it is rare. 



The following table briefly indicates infectious diseases in which 

 permanent immunity follows an attack : 



Infectious Diseases in Which One Attack Conveys Lasting Immunity 



Plague. 



Typhoid second attack rare about 2.4 per cent. (Curschmann). 



Cholera. 



Small-pox second attack very rare. 



Chicken-pox second attack very rare. 



Scarlet fever second attack about 0.7 per cent. 



Measles second attack uncommon, but less rare than scarlatina. 



Yellow fever. 



Typhus fever. 



Syphilis reinfection rare, though more common than formerly supposed. 



Mumps-second attack rare (Kraus). 



Poliomyelitis. 



13 Cornet and Kossel in "Kolle u. Wassermann," Vol. 5, 2d Ed. 



