I 2 INTRODUCTION 



ease by inoculation with blood of diseased animals was already 

 known. Koch was able to grow the bacillus in pure culture in a 

 test tube, using the aqueous humor of the ox's eye as a medium. 

 He was able to observe growth and division and the formation 

 and germination of spor.es under the microscope. Finally with 

 these cultures, which had been prop9,gated a long time in the 

 culture medium, he was able again to cause anthrax by injecting 

 them into susceptible animals. The demonstration of the causa- 

 tion of disease by bacteria had been achieved. 



The introduction by Koch in 1881 of the plate method of sepa- 

 rating bacteria paved the way for rapid advances in bacteriology, 

 and. during the next ten years the bacterial causes of several 

 diseases were discovered and proven by thorough test, and since 

 then the number of diseases known to be due to bacteria has 

 gradually increased. 



The history of immunity extends far back into ancient times. 

 For many diseases it was recognized that those who recovered 

 could associate with the sick without danger to themselves. 

 Recognizing this, people sometimes exposed themselves purposely 

 in order to have the disease at a convenient time. Artificial 

 inoculation to cause small-pox was introduced into Europe from 

 the Orient in 1721. The use of cowpox, vaccination, was discov- 

 ered by Jenner in 1797. Artificial immunization by inoculation 

 with altered bacterial cultures was first successfully demonstrated 

 by Pasteur in chicken cholera and in anthrax in 1881. The 

 practice of inoculation with dead bacterial cultures has' become 

 almost universal in the armies of the world since 1914, for the 

 prevention of enteric fevers. Analogous methods have been 

 devised for many other diseases. The discovery of the antitoxic 

 property of the blood serum of animals immunized to tetanus 

 and to diphtheria was made by von Behring and Kitasato (1891). 



With the discovery of amebae in the stools in tropical dysentery 

 by Loesch (1875) and of the malarial Plasmodium in the blood 

 by Laveran (1880) the relationship of protozoa to important 

 diseases was suggested. An enormous number of protozoal para- 



