212 INTRODUCTION TO BOTANY 



both diseased and healthy twigs, the knife may be the means 

 of transferring bacteria. If all infected parts are removed and 

 burned, and if the knife used in pruning diseased twigs is 

 sterilized before being used in pruning healthy plants, the 

 continued spread of the disease is made unlikely. 



197. Diseases of man. In the section on nutrition of the 

 bacteria (sect. 188) attention was directed to the fact that ex- 

 cretions are regularly produced by them. In case of disease- 

 producing forms, some, of these excretions are injurious or 

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

 animals toxins may produce disease. Each kind of disease- 

 producing bacterium forms its own peculiar toxin or toxins, 

 which in time may produce a particular kind of disease. Sub- 

 stances that neutralize toxins are known as antitoxins. In the 

 body of the diseased organism antitoxins are produced which, 

 when formed in sufficient quantities, counteract the influence 

 of the toxins. When one has had an attack of smallpox or 

 diphtheria and has produced sufficient antitoxin to enable him 

 to overcome it, he is usually not susceptible to another attack 

 of the same disease. He is immune. There are other diseases 

 (such as mumps, measles, and scarlet fever) against which 

 most people may acquire immunity by once surviving an at- 

 tack. This immunity is usually lifelong, though exceptions 

 are known. In the case of other diseases (such as typhoid and 

 influenza), one may become susceptible to another attack. 

 Some people are naturally immune to certain diseases. 



Smallpox vaccination 1 consists in infecting human beings 

 with organisms that have been grown in such unfavorable 

 ways that their ability to produce disease is greatly reduced. 

 Consequently the result of vaccination is to cause a very mild 

 attack, which produces immunity against fully virulent small- 

 pox. This lasts for a period of years (usually given as seven 

 years), though the protective effect gradually diminishes. 



1 The specific organisms which produce smallpox have not been fully 

 identified. It is thought by some that smallpox is caused by an animal 

 organism somewhat like the malarial parasite, and not by bacteria. 



