172 PEACTICAL BOTANY 



158. Relation to diseases of man. In the section on nutri 

 tion of the bacteria (Sect. 153) attention was directed to the 

 fact that excretions are regularly produced by them. In case 

 of disease-producing forms some of these excretions are inju- 

 rious or poisonous, and are known as toxins. In susceptible 

 plants or animals toxins may produce disease. Each kind 

 of disease-producing bacteria forms its own peculiar toxin or 

 toxins, which in time produce particular kinds of disease. 

 Substances that neutralize toxins or their effects are known 

 as antitoxins. The diseased organism tends to manufacture 

 these " anti-bodies," or antitoxins, which, when formed in 

 sufficient quantities, counteract the influence of the toxins. 

 In some cases (smallpox, diphtheria), when one has had a 

 disease and has produced sufficient antitoxin to enable him to 

 overcome the attack, he is usually not again susceptible to 

 this particular disease. He is immune. Some people are natu- 

 rally immune to certain diseases. There are diseases (such as 

 mumps, measles, scarlet fever, diphtheria, smallpox) against 

 which most people may acquire immunity by once surviving 

 an attack. This immunity is usually lifelong, though excep- 

 tions are known. In the case of other diseases (typhoid, in- 

 fluenza), one is soon susceptible to another attack. 



Smallpox vaccination 1 consists in infecting human beings 

 with bacteria that have been grown in such unfavorable ways 

 that their ability to produce the disease* is greatly reduced. 

 Consequently the result of vaccination is to cause a very mild 

 attack, which, however, is strong enough to produce immunity 

 against fully virulent smallpox. This lasts for a period of 

 years (usually given as seven years), though the protective 

 effect gradually diminishes. 



In the case of diphtheria it has been found possible to secure 

 from .horses and mules an antitoxin that will counteract diph- 

 theria toxins in the human body. These animals are naturally 

 immune to diphtheria, but by injecting into their bodies toxins 

 produced in beef broth by diphtheria bacteria, this natural 



1 The specific bacteria which produce smallpox have not been identified. 



