674 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMaIs 



The Factors which Influence the Results of an Infection 



There are four principal factors which influence the results of an 

 infection. 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 plant, 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 

 example, Strept. pyogenes may infect the skin of mucous membranes 

 and produce only an abscess of varying proportions. Again, it may be 

 more virulent. The resistance of the infected individual may be 

 lowered somewhat and the streptococcus may enter the lymphatics 

 of the skin and produce erysipelas or the blood stream and produce 

 a fatal septicemia. Furthermore, one strain of the streptococci in 

 the blood may produce a very virulent infection and another a less 

 severe one. Virulent streptococci are not phagocytized by the leu- 

 cocytes. The same variation in virulence is noted in all the pathogenic 

 bacteria and the infections are modified thereby. The fact that an 

 organism is virulent for an animal is not evidence that it is virulent for 

 man. The virulence of an organism depends upon its ability to form 

 toxins or other poisonous substances. 



Number. — The number of infecting microorganisms which are 

 introduced into an animal body is of importance. In anthrax, for 

 example, it has been shown that one bacterium is capable of multiply- 

 ing and setting up an infection. In, tuberculosis and typhoid fever and 

 most of the infectious diseases it requires a rather large number before 

 an infection will take place. The leucocytes, bactericidal substances, 

 in the blood, and the body cells in general are capable of destroying many 

 infectious agents. Furthermore, it can be readily understood how a 

 few bacteria might be able to cause a mild infection and an increasing 

 number be able to so overcome the bodily resistance as to cause a more 

 or less severe infection. 



Avenue. — It has been pointed out previously how the avenue of 

 iiifection modifies the infection. A very virulent microorganism may , 

 occasionally produce a very mild infection when introduced in a 

 certain locahty while in another place the same organism may produce 



