280 FLIES AND DISEASE 



remarkable achievements already attained in con- 

 trolling and eradicating some of the most deadly 

 pestilences. Success in controlling an epidemic 

 disease transmitted by the agency of insects depends 

 on a thorough knowledge of the various factors 

 concerned in its production : the life-history of the 

 causative parasite in man; the periods during 

 which he is capable of infecting the insect carrier; 

 the influence of drugs on the parasite while in the 

 human body; the relationship of the parasite to 

 the transmitting insects; the life-histories of the 

 insects and their habits under various conditions; 

 and the influence of climate, locality, etc., on the 

 patients, the parasites and the insects. 



In regard to most diseases the necessary know- 

 ledge can be acquired only by slow degrees. First 

 the disease is separated from others which resemble 

 it by careful analyses of the symptoms prevailing in 

 various outbreaks. In time it is recognized that 

 patients exhibiting certain groups of symptoms are 

 suffering from a definite disease, which is probably 

 caused by a certain kind of microorganism. Until 

 considerable advances in bacteriology had been 

 made the identification of some infectious diseases, 

 and especially of the less common forms of these 

 diseases, was very uncertain. Not only have 

 bacteriology, concerned mainly with those minute 

 forms of life, which are regarded as vegetable in 

 type, and the allied science of protozoology, which 

 deals with the minute forms, which are regarded as 



