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 emmune. 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! 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. 
