SUSCEPTIBILITY AND IMMUNITY. 243 



individual; that in others it is more or less temporary, as shown by 

 the occurrence of a subsequent attack. 



The protection afforded by a single attack not only differs in dif- 

 ferent diseases, but in the same disease varies greatly in different 

 individuals. Thus certain individuals have been known to suffer 

 several attacks of small-pox or of scarlet fever, although, as a rule, a 

 single attack is protective. Exceptional susceptibility or insuscepti- 

 bility may be not only an individual but a family characteristic, or 

 it may belong to a particular race. 



In those diseases in which second attacks are not infrequent, as, 

 for example, in pneumonia, in influenza, or in Asiatic cholera, it is 

 difficult to judge from clinical experience whether a first attack exerts 

 any protective influence. But from experiments upon the lower ani- 

 mals we are led to believe that a certain degree of immunity, lasting 

 for a longer or shorter time, is afforded by an attack of pneumonia 

 or of cholera, and probably of all infectious diseases due to bacterial 

 parasites. In the malarial fevers, which are due to a parasite of a 

 different class, one attack affords no protection, but rather predis- 

 poses to a subsequent attack. 



In those diseases in which a single attack is generally recognized 

 as being protective, exceptional cases occur in which subsequent 

 attacks are developed as a result of unusual susceptibility or expo- 

 sure under circumstances especially favorable to infection. Maiselis 

 (1894) has gone through the literature accessible to him for the 

 purpose of determining the frequency with which second attacks 

 occur in the various diseases below mentioned. The result is as 

 follows : 



Second Third Fourth , . , 



Attacks. Attacks. Attacks. M - 



Small-pox 505 9 514 



Scarlet fever 29 4 33 



Measles 36 1 37 



Typhoid fever 202 5 1 208 



Cholera 29 3 2 34 



These figures support the view generally entertained by physi- 

 cians that second attacks of scarlet fever and of measles are compar- 

 atively rare, while second attacks of small-pox are not infrequently 

 observed. Considering the very large number of cases of typhoid 

 fever which occur annually in all parts of Europe and America, the 

 number of second attacks collected does not bear a very large propor- 

 tion to the total number taken sick, although the recorded cases, of 

 course, fall far short of the total number of second attacks of this 

 and the other diseases mentioned. 



The second attacks of cholera recorded are not numerous, and, no 

 doubt, a carefullly conducted investigation made in the areas of en- 



