236 THE FUNGI 



human or other host. They are contagious because the germs 

 can be easily passed directly or indirectly in various ways from 

 the ill person to those around him. 



The active substances which affect the patient are known in 

 all cases to be certain poisons called toxins, which are, for the 

 most part, secretions, less often decomposition products, accom- 

 panying the growth of the bacteria. These poisons become dis- 

 tributed by the blood and cause the fevers. The body resists 

 the effects of the toxins to the best of its ability, and in some 

 cases substances are formed called antitoxins, which neutralize 

 the poisons. The injection into the human body of an antitoxin, 

 which is obtained from the blood of a horse infected with diph- 

 theria, is the chief principle in the " antitoxin " treatment of this 

 very serious disease. Recovery from a germ disease generally 

 renders the person safe, or immune from further attack for a 

 long time, because the body has developed resistant powers 

 to the poisons and growth of that particular germ. The viru- 

 lent poisons called ptomaines are usually the result of bacterial 

 growths in foods that have not been properly kept. 



Inflammation of wounds is caused by germs, and the forma- 

 tion of pus is in large part the gathering of white blood corpus- 

 cles which feed on the germs as they multiply i& the infected 

 tissues. The whole practice of modern surgery is based on 

 absolute cleanliness in the treatment of wounds to prevent the 

 entrance of bacteria during operations. 



There are some serious bacterial diseases of plants, as the 

 pear and apple blight, cucumber and melon wilt, black rot of 

 cabbage, wet rot of potatoes, and hyacinth blight, and probably 

 peach yellows is also of this class. 



258. Public health. The matter of public health and 

 hygiene calls for constant attention on the part of physicians 

 and health officers to the possible sources of germ diseases. 

 For example, contaminated water and impure milk are the 

 commonest means of infection for typhoid fever, and epidemics 

 of this disease are frequently traced to these sources. We 



