8ERUM RESISTANCE OF ORGANISMS. Ill 



progresses steadily. Supposedly, the natural anti- 

 bacterial forces of the serum and leucocytes took 

 up the destruction of the bacteria until the former 

 were exhausted, and from this time proliferation 

 could take place with little hindrance. In infec- 

 tions which occur naturally, as in typhoid fever or 

 scarlet fever, the first organisms which reach the 

 underlying tissues and circulation may be 

 destroyed, involving such a loss of bactericidal 

 agencies that continued proliferation and invasion 

 finally results in general infection. 



In speaking of passage, it has already been "Serum 

 stated that virulence may be increased by permit- 

 ting an organism to grow in the tissues of a living 

 animal. Virulent organisms are not taken up by 

 leucocytes so readily as avirulent, and in the pres- 

 ence of high virulence there may be no phagocytosis 

 at all. This has a bearing on the incubation period 

 in that the micro-organisms which first come in 

 contact with the tissues of the host may have low 

 virulence, but after exposure to the germicidal 

 substances for a sufficient length of time, their 

 pathogenicity and resistance may be so raised that 

 they escape destruction in the tissues. On this 

 basis, therefore, the incubation period may, in part, 

 represent the time required for the organisms to 

 become serum-resistant. 



Regarding the subject under discussion, there is 

 still another factor which finds application, par- 

 ticularly where toxins having an enzyme-like 

 nature, are involved. Toxins have been likened to 

 enzymes, because of their action in exceedingly 

 small doses, their common susceptibility to heat, 

 and finally the exhibition of an incubation period. 

 No matter how much diphtheria or tetanus toxin 



