PARASITES 423 



acceptance and it was the dominating theory of disease through- 

 out the whole of the dark ages. Indeed, its influence is still 

 potent in much of the current thought and practice in medi- 

 cine today. By the giving of drugs of many kinds for many 

 kinds of diseases, a large number of really valuable medicines 

 which are in wide use today were thus empirically discovered, 

 i.e., discovered by experiment. Quinine, for example, was 

 known to be a cure for malaria long before it was known 

 that the disease is caused by a little microscopic animal which 

 is killed by the quinine. 



It must be some little surprise to you to realize that it was 

 not until a comparatively recent date when the GERM THEORY 

 OF DISEASE began to be generally accepted that the practice 

 of medicine, really began to be placed on a sound scientific 

 basis. The year 1876 is a memorable year in the history of 

 medicine for it was in that year that Robert Koch succeeded 

 in demonstrating beyond the possibility of a doubt that a 

 certain disease of domestic animals was caused specifically 

 by a certain rod-shaped bacterium which is now known as 

 BACILLUS ANTHRAX. Since this is a typical bacterial disease 

 and one about which a great deal is known, we shall use it to 

 illustrate the nature of a bacterial disease. 



ANTHRAX 



488. Animals Affected. ANTHRAX, or SPLENIC FEVER, as it 

 is sometimes called, is, in nature, primarily a disease of cattle 

 and sheep though it sometimes attacks a large number of 

 other animals including man. Horses, hogs, and dogs have a 

 relatively high degree of resistance to the disease though none 

 of them is entirely free from occasional attacks of it. Rab- 

 bits, guinea-pigs, and white mice are extremely susceptible, 

 especially so when the bacteria are injected under the skin 

 of the animal. A single bacterium injected under the skin 

 of a white mouse is sufficient ultimately to cause the death 

 of the animal. Carnivorous, or flesh-eating animals are, as a 



