320 ANIMAL BIOLOGY 



who did more than anyone else to establish the germ theory 

 of disease, was led to his most important discoveries on 

 account of his previous studies of the phenomena of fer- 

 mentation and putrefaction. Once the germ theory of 

 disease was established, contagion was no longer a mystery, 

 but a natural result of the transfer of minute organisms 

 from one person to another. Just as a drop of decaying 

 substance will set up decomposition in fresh material, so 

 will a small amount of matter from a diseased person con- 

 vey the disease to a healthy individual. It has been 

 shown that, in a great many diseases, bacteria are uni- 

 formly present in great numbers in the tissues of the person 

 affected. In many cases it has been found possible to 

 cultivate the germs of certain diseases in artificial media 

 outside the body, and to propagate them free from admix- 

 ture with other germs. The germs from such "pure 

 cultures," as they are called, have been shown to give 

 rise to the disease in question when inoculated into the 

 body of a healthy person.. 



When the secret of contagion was known, it became 

 much easier to check the spread of contagious diseases. 

 For this purpose much use is made of germicides and dis- 

 infectants, substances which kill the germs of the disease. 

 Rooms that have been occupied by diseased persons are 

 commonly fumigated with formaldehyde, a strong germi- 

 cide, before they are again occupied. Articles used in 

 connection with the patient are washed in an antiseptic 

 solution or boiled. And the patient is so far as possible 

 kept free from contact with healthy people until disease 

 germs are no longer given off from his body. 



Formerly surgical operations were commonly attended 

 with gangrene, blood poisoning and other infections 

 which we now know are caused by bacteria. Surgeons 

 now exercise the greatest care in keeping the wounds of 



