282 FLIES AND DISEASE 



this fact that the modern measures for the control 

 of such diseases have been founded. 



The minute parasites which cause disease may 

 be divided into three classes, the bacteria, the 

 protozoa and the ultra-microscopic organisms. 



Innumerable organisms belonging to all three 

 classes exist, but only a small proportion of them 

 are in any way connected with disease, the majority 

 performing various other functions in the economy 

 of nature. Several species of bacteria produce 

 diseases of different kinds, some when they gain 

 entrance into the intestinal tract, and others when 

 they invade the tissues. In most bacterial diseases 

 the organisms are conveyed directly from a patient 

 or carrier to a healthy person, while in some these 

 organisms may be conveyed in milk, water, food, or 

 by insects, and certain species, which are present 

 in the soil, are apt to infect wounds. Bacteria 

 multiply by division when they find conditions 

 suitable for their growth, and exhibit no complicated 

 phases in their life-cycles. Most species of disease- 

 producing bacteria are easily killed by exposure to 

 light and by drying, and apparently do not survive 

 very long outside the bodies of men or animals. 

 A few species however are more resistant, especially 

 such as are capable of forming spores. So far 

 as we know at present most disease-producing 

 bacteria do not multiply in the bodies of insects, 

 or live in them for more than a few days. In 

 regard to bacteria, therefore, flies only act as 



