124 IMMUNITY, NATURAL AND ACQUIRED 



a protective effect against subsequent attacks of the same disease, 

 either completely or partially, as regards that individual, e.g.j 

 an attack of small-pox which has been recovered from, 

 guarantees immunity from another attack throughout a life-time ; 

 e.g., also measles, scarlet fever, syphilis, yellow fever, whooping 

 cough — while attacks of typhoid fever, pneumonia, and diphtheria 

 only confer a temporary insusceptibility. This partial immunisa- 

 tion is also shewn in races as well as individuals, c.//., measles has 

 been endemic among European races probably for centuries, 

 and as a consequence an ordinary attack of measles in a child 

 is looked upon as a trivial and inevitable ailment, thanks to the 

 partial immunity handed down from long series of ancestors 

 who have passed through the illness, but let this comparatively 

 trivial disease be imported into a community where the disease 

 has been unknown, and the result is a virulent epidemic with 

 an appalling mortality, e.ij., the introduction of measles among 

 the Fijians some 35 years ago resulted in a mortality of nearly 

 40,000 of the native race. Similarly chicken-pox, a trivial 

 disease of children, becomes almost as deadly as small-pox 

 among coloured races when first introduced. Our experience in 

 Australia shews that typhoid fever has only a partial protecting 

 power, as most medical men can point to cases where two or 

 three or even more attacks have been sustained by the same 

 individual. In India, similarly, plague has been shewn to 

 affect an individual on two or more occasions. 



(B) It is in an attempt to imitate this method of Nature 

 that man has devised various ways of giving a slight dose of the 

 disease to protect from severer attacks. This was recognised 

 long ago in the introduction into England of inoculation with 

 the scabs of small-pox, which set up a mild attack of small-pox. 

 A safer method, because more under control, is the — 



(rt) Inoculation with attenuated virus, i.e., with living 

 oro-anisms of a low degree of virulence. It must be known to 

 all of you that bacteriologists have long since demonstrated the 

 possibility of raising or lowering the virulence of various organ- 

 isms by sslection of cultures, or by what is the same thing, 

 selection of animals through which to pass the micro-organistn- 

 Practically every organism when cultivated for some time 

 outside the body loses its virulence, and in the case of some 

 this is very marked indeed, c.//., pneumococcus. Pasteur found 

 ii the ease of chicken cholera that when cultures were kept 

 for a long time under ordinary conditions, they gradually lost their 



