6 BACTERIA, YEASTS, AND MOLDS 



It is hardly surprising that some one in these eariy days 

 shoiild have guessed that these minute creatures, which must 

 enter our bodies with our food and drink, may have been the 

 cause of disease. In fact, the first guess of this kind was 

 made as early as 1546 by one Fracastorio of Verona, who had 

 never seen any microscopic organism. In spite of this fact, 

 he proposed a germ theory of disease quite like what we 

 believe to-day, except that he had no idea what sort of crea- 

 tures his "living contagious bodies" might be. Two centu- 

 ries later (1762) an Austrian physician, Plenciz, was able to 

 go further than this, for it was known by that time that 

 microorganisms really existed and that there was more than 

 one kind of them. He suggested, in fact, that each disease 

 might have its own specific microscopic agent which could 

 cause that disease and no other. He explained that these 

 microscopic bodies could be distributed through the air, 

 enter the body and Hve there, producing the disease, and 

 believed that they might then spread to other hosts by means 

 of discharges from the body. Plenciz's theory was not gen- 

 erally accepted in those days. It was, of course, based on 

 pure speculation; and neither he nor his followers could 

 obtain evidence to prove it. So the theory fell into dis- 

 repute; and as late as 1820 it was casually referred to as 

 an " exploded theory." 



The Origin of Life 



When a housewife cans a lot of vegetables or fruits, she is 

 confident that if the canning is well done, the contents of the 

 can will not spoil. If she is intelligent, she knows that 

 means that she has killed the microscopic organisms present 



