72 BACTERIOLOGY 



heat, depression, and other unknown influences, increase 

 the susceptibility to intestinal infection. 



TRANSMISSION OF INFECTION 



With the exception of the tetanus bacillus, which lives 

 naturally upon the soil and the intestinal contents of 

 horses, most pathogenic bacteria can exist but a short 

 time outside the animal body. Transmission of infec- 

 tion usually occurs by direct contact with an individ- 

 ual having the disease or with a secretion from his 

 body. 



The gonococcus dies very soon outside the body, the 

 pneumococcus is longer lived, and the typhoid bacillus 

 and tubercle bacillus can exist for a considerable length 

 of time outside the body. The diphtheria bacillus and 

 typhoid germ even grow and develop in milk. Not a few 

 epidemics have been traced to this source. Recent in- 

 vestigations tend to show that the germ carrier is re- 

 sponsible for the transmission of many cases of infection. 



Carriers are individuals who after recovery from disease 

 continue to throw off virulent germs for long periods of 

 time, or others who, after contact with infected persons, 

 carry the germs in their nose, throat, or intestines, though 

 themselves healthy. Among the diseases thus trans- 

 mitted are typhoid fever, cholera, influenza, pneumonia, 

 and diphtheria. 



Germs which leave the body with intestinal excretions, 

 as in typhoid and cholera, are likely to contaminate a 



