Disease Transmission and Mans Resistance 309 



tended to affect the same person repeatedly. Once tfie germ 

 theory of disease had been estabhshed, a number of attempts to 

 explain these differences in resistance were proposed, but the 

 following four theories received more than passing interest at one 

 time or another. 



Pasteur's Exhaustion Theory 



The more one studies the history of microbiology, the more he 

 is impressed with the diversity of topics with which Louis Pasteur 

 concerned himself. He was a brilliant scholar, and while sub- 

 sequent research in the light of new knowledge disproved some of 

 Pasteur's ideas, his fertile mind proposed lines of investigation 

 faster than people were able to study them. With respect to im- 

 munity, he felt that the host probably harbored some substance 

 vital to the growth of given organisms. Once the microbes had 

 exhausted the supply of this required substance, the invading mi- 

 croorganisms died or left the host. Since this same species of 

 microorganism could not survive in the absence of this essential 

 substance, further infection was impossible in that particular host. 

 In other words, he was immune. While interesting historically, 

 this theory of immunity has no scientific foundation. 



Metchnikoff's Cellular Theory 



As early as 1870 it had been observed by several investigators 

 that white blood cells (leucocytes) sometimes harbored bacteria, 

 but it was not for nearly fifteen years after this observation that 

 Elie Metchnikoff put forth his concept of immunity based upon 

 the reaction which is called phagocytosis. Phagocytes, which 

 means devouring cells, are certain types of white blood cells which 

 are capable of ingesting and destroying bacteria, but this devouring 

 ability is increased in the presence of an antibody called opsonin, 

 which prepares the bacteria for engulf ment by the leucocytes. 

 When PUS is examined under a microscope it is found to consist of 

 bacteria engulfed by white blood cells, serous fluid, and debris. 

 While many invading bacteria may meet their doom as the result 



