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. Total. 
Small-pox............. ... 505 9 0 514 
Scarlet fever wpa 29 4 0 33 
Measles........... a 86 1 0 37 
Typhoid fever............... 202 5 1 208 
Choleray ss .is..054 neuesacen ce 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- 
