ACQUIRED IMMUNITY 61 



No lasting immunity is conferred by one attack in: 



Infection with the Pyogenic cocci 

 Gonorrhea 

 Pneumonia 

 Influenza 

 Glanders 

 Dengue fever . 



Diphtheria in general protection, second attack in 0.9 

 per cent, cases 0.01 antitoxin unit per c. c. of circu- 

 lating blood protects. 

 Recurrent fever 

 Tetanus. 

 Erysipelas. 

 Beri beri 

 Malaria . 

 Tuberculosis . 



These observations actually form the point of departure of that 

 entire branch of medical science which devotes itself to the study of 

 resistance to infection, serum diagnosis, and specific therapy, and 

 it will be seen that all the facts that have been gathered upon these 

 subjects are the fruits of detailed analysis of this phenomenon of 

 acquired immunity. 



Its occurrence in many instances has been so striking that 

 ancient observers, long before the birth of rational medicine, referred 

 to it, and often drew from it conclusions of great hygienic impor- 

 tance. Thucydides, in the second book of his account of the Pelopon- 

 nesian Wars, in describing the plague at Athens, notes the apparent 

 safety from reinfection of those who had recovered, suggesting the 

 possibility of their being therefore immune against disease in general. 

 The literature of the Middle Ages and of earlier modern times 

 contains numerous further references which indicate that acquired 

 resistance was clinically recognized as a result of recovery from 

 many diseases. The phenomenon was not only observed, but put to 

 practical utilization by the ancients of China and India. Thus the 

 practice of inoculating children with small-pox material from the 

 active pustules of patients, or making them sleep in beds or wear 

 the shirts of sufferers was a dangerous practice but logical, on the 

 reasoning that the disease conveyed to a person in full health and 

 good condition would probably take a mild course and confer im- 

 munity, while the naturally acquired disease, contracted often be- 

 cause of the weak and debilitated condition of the individual, would 

 be more apt to end fatally. 



Such methods, though barbaric and eventually unjustified by the 

 naturally high mortality incident upon them, were actually brought 

 to Europe from the East, and for a time practiced in European 

 countries. 



The first great advance which bridged the gap between the obser- 



