MAN'S FIGHT FOR SURVIVAL 621 



in India, a third in Arabia, and a fourth in the interior of Africa, to 

 which must now be added a fifth area on our western coast. Plague 

 is really a disease of rats and ground squirrels, but through the 

 activity of fleas it can be transferred from a sick rat to the body of 

 man, where it thrives. Over a million rats were killed in fighting 

 the last outbreak of bubonic plague in California and great care has 

 to be used in quarantine to prevent rats from reaching our shores 

 through ships from countries where the plague is endemic. 



The Relation of Bacteria to Disease 



Bacteria are present almost everywhere as parasites. They are 

 found inside as well as outside of the human body, existing in countless 

 milHons in the mouth, on the teeth, and particularly in the lower part 

 of the food tube. There has been a good deal of discussion as to 

 whether the bacteria in the food tube are harmful or useful. Experi- 

 ments indicate that in some animals, at least, bacteria live as mess- 

 mates in the digestive tract, actually helping the host by breaking 

 down waste from foods. Several recent experiments have shown that 

 intestinal bacteria are not necessary, however, in the life process of 

 the host. 



Certain Bacteria Called Pathogens Cause Disease 



These organisms, like other living things, take in food and form 

 organic wastes within their own bodies which they give off as toxins. 

 Toxins that diffuse through the body tissues of the host where the 

 infection occurs are called exotoxins, while those retained within the 

 bodies of the bacteria to be released at their death are referred to as 

 endotoxins. Every species of pathogenic bacteria forms a particular 

 toxin which has a specific action on the host, frequently causing symp- 

 toms of a definite disease. When bacteria die, as they may in great 

 numbers during the progress of a disease, they break down, releasing 

 protoplasmic constituents that separate from each other, splitting into 

 smaller and smaller molecular groups as proteins do when changed 

 to amino acids during digestion. These split proteins, as they are 

 called, may be extremely poisonous and act in many cases as toxins. 

 Bacteria also break down body tissues of the host, in some cases 

 destroying the intestinal lining, blood corpuscles, or, as in tuber- 

 culosis, definite tissue cells. Parasitic bacteria that cause boils and 

 abscesses are believed to send out enzymes which dissolve the white 

 corpuscles so that they may be used by bacteria. 



