532 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



plant and animal body. The variation in the route produces a great 

 variation in the type and the results of the infection. There is some 

 variation in what constitutes an infection, depending upon the infectious 

 microorganism and the tissue it attacks. For example, Msp. comma, of 

 Asiatic cholera, does not produce an infection unless it comes in contact 

 with the intestinal mucosa and in this case it does not enter the tissues 

 but sets up an inflammatory process on the surface. If this same micro- 

 organism comes in contact with tissues such as those of the nose, throat, 

 and lungs, no infection results. In the case of B. typhosus, the bacillus only 

 attacks the intestinal mucosa, but in addition it enters the tissue of the 

 lymphatic patches and sets up an inflammation. This microorganism 

 may invade the circulatory system directly. Again, in order for such an 

 organism as the Strept. pneumonia; to produce pneumonia it is only 

 necessary for the bacteria to come in contact with the thin, single-celled 

 alveolar wall through the air passages. In case this bacterium produces 

 an abscess it is necessary for it to first enter into the tissues. In the 

 pneumonic form of plague the Bact. pestis is carried to the alveolus 

 through the circulation and thus enters the tissues of the lung before 

 actually invading the alveoli. This sometimes occurs in case of Strept. 

 pneumonia. It also gives rise to abscesses but only when it invades 

 lymphatic glands. The same is true of the large number of infectious 

 microorganisms: there is a variation in the infection due to the variation 

 in the microbic agent and the point where this agent attacks the body. 

 The severity of an infection, as for example, a pneumonia due to Strept. 

 pneumonia, or to Strept. pyogenes; or to Bact. pneumonia (Friedlander), 

 or to Bact. pestis, etc., would vary with the infectious agent, its virulence 

 and number, and with the resistance of the individual infected. 



THE FACTORS WHICH INFLUENCE THE RESULTS OF AN INFECTION. 



There are four principal factors which influence the results of an in- 

 fection. They are as follows: the virulence of the infecting micro- 

 organism; the number of the infecting microorganisms; the avenue by 

 which the infectious microorganism enters the body; and the resistance 

 of the animal or individual infected. 



VIRULENCE. It is a self-evident fact that the more virulent a micro- 

 organism is the more serious will be the infection which results from its 

 invasion of the body. There is a great difference in the virulence. For 



