PHENOMENA OF INFECTION. 71 



When it has this power it is said to be virulent. In 

 another chapter the remarkable inconstancy of the 

 toxicity of micro-organisms has been considered (vide 

 p. 41). That virulence in bacteria is an essential 

 phenomenon in disease-production has been abundantly 

 confirmed by the finding, in their propitious channels 

 of entry, of various pathogenic micro-organisms in 

 perfectly healthy individuals, the organisms so found 

 being commonly non-virulent varieties. 



All pathogenic bacteria vary in respect to virulence, 

 and, as has been explained in a previous chapter, are 

 on this account divided into those which are virulent 

 and those which are non- virulent. However, it should 

 be remembered that non-virulent bacteria are not 

 without danger to individuals who carry them, and to 

 other persons to whom they may be conveyed, since 

 they may acquire virulence either before or after trans- 

 ference, and provoke disease. 



Microbic association in disease is of 

 MicROBic common occurrence, the resulting mixed 

 Association infection usually being severer than an 

 (symbiosis), unmixed one. Thus in diphtheria, the 



diphtheria bacillus is often associated 

 with the streptococcus pyogenes or the staphylococcus 

 pyogenes aureus; the typhoid bacillus with the bacillus 

 coli communis^ a common inhabitant of the intestinal 

 tract. There are some bacteria, however, which cannot 

 exercise their power without the co-operation of another 



