METHODS AND CHANNELS OF INFECTION. 533 



example, Strept. pyo genes 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 some- 

 what, the organism enter the lymphatics of the skin and thus produce 

 erysipelas, or the blood stream and produce a fatal septicaemia. Further- 

 more, one strain of the streptococcus in the blood may produce a very 

 virulent infection and another a less severe one. Virulent streptococci 

 are not phagocytized by the leucocytes. The same variation in viru- 

 lence is noted in all the pathogenic bacteria and the infections are modi- 

 fied 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 along with other factors upon its ability to form toxins or pois- 

 onous substances. 



NUMBER. The number of infecting microorganisms which are intro- 

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

 it has been shown that one bacterium is capable of multiplying and 

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

 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 

 microorganisms. 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 very severe 

 infection. 



AVENUE. It has been pointed out previously how the avenue of 

 infection modifies the infection. A very virulent microorganism may 

 occasionally produce a very mild infection when introduced in a certain 

 locality while in another place the same organism may produce a very 

 severe type. The results of the infection will be materially modified, 

 therefore, depending on the avenue of entrance which the virulent 

 microorganism takes. For example, in addition to those mentioned 

 previously, suppose Bad. pestis, the causal agent of plague, enters the 

 blood through the skin or the lymphatics through the tonsils; it is 

 carried to the lungs and there produces a very severe pneumonia; if 

 the bacteria enter the lymphatic system in large numbers they fre- 

 quently localize in the lymph glands producing buboes or glandular 

 enlargements not always fatal. They may also enter the blood current 

 and produce a rapidly fatal septicaemia. It is not established that the 



