532 Introduction to the Study of Science 



Vaccination is actively preventive of smallpox, and immunity 

 to typhoid fever is acquired by a similar method. Diphtheria 

 is treated successfully by inoculation with diphtheria serum 

 (see page 606) ; and within recent years certain drugs have 

 been discovered which are specific for several common and 

 dangerous infectious diseases. 



Second. Simple but adequate preventive measures can be 

 employed by everyone to block or destroy the animal agencies 

 or other media of transmission of pathogenic germs. Follow- 

 ing the discoveries by Ross, scientific investigation has proven 

 that several dangerous diseases are caused by animal or plant 

 parasites (pages 588 ff.) which are distributed by insects. Other 

 varieties of the mosquito are known to spread yellow fever and 

 dengue or break-bone fever ; the common or domestic fly is a 

 carrier of the typhoid fever germ ; the tsetse fly spreads sleeping 

 sickness; while fleas, bedbugs, ticks, and lice are carriers of 

 germs causing cholera, bubonic plague, spotted fever, and typhus 

 fever. Although many diseases have not been thoroughly 

 investigated and their causes discovered, it is certainly a wise 

 method of procedure to employ adequate measures to prevent 

 the transmission of any disease by insects, other animals, 

 utensils, clothing, food, water, or other possible agencies. 



261. Yellow fever. In the year 1881 Dr. Carlos Finlay 

 suggested that yellow fever may be transmitted from an in- 

 fected person to a healthy one by a mosquito. He failed to 

 prove this by his experiments, because he did not know that a 

 mosquito after feeding on the blood of a patient does not 

 become infective under a period of twelve or more days. This 

 fact was discovered by others, and Dr. Finlay's hypothesis 

 was proven to be true by a series of practical tests. 



The triumph of scientific knowledge over the cause and agency 

 of this disease has been witnessed in many localities. In the 

 southern states, in Havana, and in the Panama Canal Zone, 

 where yellow fever had held full sway, there is now scarcely a 

 trace of the disease. This is due to the extermination or control 



