622 MAN AS A CONQUEROR 



Like other parasites that have been mentioned, bacteria show con- 

 siderable variation as to choice of host. Some few, such as those 

 causing typhoid, Asiatic cholera, or syphilis, are restricted to man 

 and apparently cannot gain and maintain a hold in the bodies of other 

 hosts. Another group, bubonic plague, anthrax, rabies, and glanders, 

 that normally live in other hosts than man, have become adapted to 

 his body through his contact with lower animals. One of the best 

 examples of accidental parasitic attack on man is bubonic plague, 

 which came through the introduction of the rat as a hanger-on m 

 homes. A third group of bacteria which includes the tubercle bacilli 

 as well as the group of the streptococci and pneumococci appear to 

 live in several different hosts. Certain of the cocci are parasitic in 

 other animals as well as in man. The bovine tubercle bacillus may 

 live in the pig or in man as well as in its original cattle host. It was 

 this habit among certain types of bacteria of living in a variety of 

 hosts that gave the clue to some of the early discoveries with reference 

 to disease. Robert Koch noticed, for example, tiny rods in the blood 

 of sheep that had just died from splenic fever. He could not afford 

 to purchase sheep to experiment with since he was a poor country 

 doctor, but he could afford mice. He found that inoculations of the 

 mice with infected sheep's blood caused the death of the mice and, 

 moreover, that the same symptoms appeared in both mice and sheep. 

 This fact led to the discovery, through the making of pure cultures, 

 that one specific germ causes the disease anthrax. Many other 

 similar discoveries have hinged on the biological factor. 



How Do Bacteria Enter the Body? 



Microorganisms causing infectious diseases enter the body through 

 some body opening, respiratory, digestive, genital, or urinary, or 

 through wounds in the skin. The most frequent means of infection is 

 through direct contact or by a spray of tiny droplets which is expelled 

 into the air while talking. Other avenues of infection are dust, which 

 spreads germ.s of tuberculosis ; impure water or contaminated milk, 

 which may contain typhoid germs ; soil, from which the tetanus bacilli 

 may be picked up ; raw foods, which may spread such diseases as 

 septic sore throat and typhoid ; and handling of articles used by 

 persons suffering from an infectious disease. In addition to these 

 means there is the introduction of infection through carriers, such as 

 insects or, in some cases, man. The human carrier, as we will see 

 later, is a most serious menace to society. 



