

Ch. X] THE BACTERIA 407 



disastrous results partly by direct destruction of tissues 

 (e.g. in tuberculosis), but in far greater degree by the de- 

 velopment, as by-products of their own metabolism, of sub- 

 stances, called toxins, which are poisonous to the host. 

 Prominent among the diseases thus caused are Diphtheria, 

 Lockjaw, Typhoid Fever, and Cholera, and there are several 

 others. Plants, too, are subject to bacterial diseases, ex- 

 emplified by Rots and Blights in cabbage, potato, and pears, 

 and by the Crown Gall, accompanied by tumor-like swellings, 

 in some common herbaceous plants. On the other hand, 

 not all deadly diseases are bacterial, for a few, notably 

 Yellow Fever and Malaria, are caused by animal parasites 

 transferred by mosquitoes ; while Infantile Paralysis is prob- 

 ably caused by another minute animal germ. Further- 

 more, some diseases are due to physiological disturbance, or 

 to other still unknown causes (pages 368-9). 



The ravages of bacterial diseases have naturally stimulated 

 the exhaustive study of these organisms themselves, and 

 encouraged the invention of methods for prevention and 

 cure. So elaborate, indeed, has become this technique that 

 the subject is now a special science, called Bacteriology. 

 Preventive methods are based mainly upon quarantine and 

 sterilization, while curative methods rest chiefly on the 

 principle of neutralizing the toxins the Bacteria form. The 

 animal body has a wonderful power of forming substances, 

 called antitoxins, which render the toxins harmless; and 

 if, in bacterial disease, the antitoxins are made by the body 

 as fast as toxins by the Bacteria, the disease is assuaged and 

 the patient recovers. These antitoxins once made, some- 

 times long persist, thus rendering the patient immune to 

 that disease. Furthermore, it is possible to transfer the 

 antitoxins from the bodies of patients, including other 

 animals which have had the disease, to a patient afflicted 

 therewith ; and this is the principle of the method of com- 

 bating diphtheria by antitoxins obtained from horses which 

 have purposely been inoculated with the disease. It is also 



