ACQUIRED IMMUNITY 75 



disease in the same individuals are not of infrequent 

 occurrence. 



That immunity may result from a comparatively 

 mild attack as well as from a severe one is a matter 

 of common observation in the case of smallpox, 

 scarlet fever, yellow fever, measles, and other infec- 

 tious diseases. And it not infrequently happens that 

 such mild attacks are not recognised. 



In that case the protection afforded during sub- 

 sequent epidemics is often ascribed to natural im- 

 munity. This is no doubt the true explanation of 

 the immunity of natives of Havana, and other cities 

 where yellow fever has prevailed for many years, to 

 this disease. An unrecognised attack suffered during 

 childhood has resulted in immunity which is supposed 

 to be due to inheritance. The popular idea that 

 natives are exempt from this disease is an additional 

 motive for calling it by some other name, especially 

 as the attacks are usually extremely mild in native- 

 born children. 



The production of immunity by protective inocula- 

 tions was for a long time limited to a single disease 

 smallpox. Inoculations with virus, obtained from a 

 pustule on a smallpox patient, were extensively prac- 

 tised before the discovery of vaccination by Jenner. 

 These inoculations gave rise to a mild attack of the 

 disease, followed by immunity, which was apparently 



