THE FRONTIERS OF MEDICINE 



disease, bacteriologists gradually defined the specific germ 

 causes of diphtheria, tuberculosis, pneumonia, typhoid 

 fever, whooping cough, scarlet fever, plague, leprosy, 

 and many other infectious diseases. There remain, how- 

 ever, a large number of diseases, which are infectious 

 in character, but for which specific causes have not yet 

 been determined. Smallpox and measles are two of the 

 common diseases of which the exact cause is not yet known. 

 Influenza and the common cold continue to be the subjects 

 of research. It has been argued that the causes of these 

 diseases are organisms that are so small that they cannot 

 be seen with the best microscope. These are the filterable 

 viruses, so-called because they will pass through a porous 

 clay filter. Much remains to be learned concerning them. 



When Paul Ehrlich discovered salvarsan, stimulus was 

 given to chemotherapeutic investigations, and innumerable 

 studies have been made in an attempt to develop specific 

 chemical substances that will attack certain germs and 

 drive them from the body. We know that certain oils, 

 such as the oil of chenopodium, are effective against the 

 hookworm; that chaulmoogra oil is specific for leprosy; 

 that salvarsan and its derivatives attack the spirochete of 

 syphilis. It remains to find specific chemical substances that 

 will attack the common pus-forming bacteria and overcome 

 invasion of the blood. 



When Almroth Wright w^ote his epoch-making study 

 on the use of vaccines, great impetus was given to the use 

 of the killed bodies of bacteria to stimulate resistance to 

 disease and to overcome infection. Now preventive vaccines 

 are available for typhoid fever. Modifications of such 

 vaccines are also applicable to other disorders. In general, 

 however, the use of vaccines as specific remedies has been 

 disappointing. 



The isolation of diphtheria toxin and the development of 

 antitoxin has established knowledge of diseases in which 



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