615. 



IV. 



Serum Therapeutics. 



IT is a matter of common knowledge that an attack of scarlet fever 

 or smallpox usually gives immunity from subsequent attack. 

 This is true of the majority of specific fevers, but not of all : it will be 

 within the memory of many that influenza confers no such privilege 

 upon its victims, and the protection given by an attack of diphtheria 

 seems to be of a very transitory nature. Of no disease is it absolutely 

 true : second attacks even of smallpox are not unknown, while in 

 measles and scarlet fever they are fairly common. It is also a 

 familiar fact that individuals vary much in their susceptibility to 

 infectious diseases ; to most, children are notoriously more susceptible 

 than adults, and the latter present wide individual variations in their 

 liability to attack. The different races of mankind vary also in their 

 susceptibility to certain infectious diseases ; in some cases there 

 seems to be a sort of natural racial immunity, in others, a relative 

 tolerance seems to have been acquired by repeated exposure to a 

 given infection for many generations, the most susceptible individuals 

 having been weeded out. Natural selection plays its part, in fact, in 

 our struggle against disease, as against other adverse circumstances ; 

 hence the mortality produced by certain of our common diseases 

 when they are introduced for the first time among uncivilised races. 



Similar facts are even more conspicuous in the susceptibility of 

 the lower animals to infectious diseases. From some human diseases, 

 such as typhoid fever and cholera, they appear exempt. Carnivora 

 are remarkably insusceptible to tubercle and to anthrax ; Algerian 

 sheep are said to possess a racial immunity from anthrax ; rodents, 

 on the other hand, are exceedingly susceptible to most infections. In 

 the case of nearly all pathogenic bacteria, it is the rule to find wide 

 variations in the virulence which they manifest upon different species 

 of animals. 



Long before any explanation of the true nature of immunity was 

 possible, attempts were made to produce it artificially in man against 

 at least one disease — namely, smallpox. By the practice of inocula- 

 tion, used for centuries in the East, and much employed in England 

 in the last century, it was attempted, by selecting a mild and 

 favourable case of smallpox from which to inoculate the healthy, to 

 produce in them a correspondingly mild attack which should confer 



