VIRULENCE 



187 



the organisation or metabolism of the host, which becomes 

 apparent as cHnical disease. 



Bacteria may enter the tissues in a variety of ways : through 

 the respiratory passages, through the tonsils, through the 

 intestinal wall or, above all, through any type of wound. 

 If saprophytic bacteria enter by any of these routes, they are 

 rapidly and effectively destroyed by the reticulo-endothelial 

 system, but a pathogenic organism is able to grow within the 

 host's tissues and its capacity to do so is a measure of its 



Fig. 13. 



" virulence." Fig. 13 shows the effect of injecting 1 million 

 streptococci into the blood-stream of a healthy animal. If 

 the organisms belong to an avirulent strain (Case 1), then 

 there is a steady decrease in the number of bacterial cells in 

 the blood-stream from the time of injection until eventual 

 disinfection. If the strain is highly virulent (Case 3), then, 

 after an initial decrease in numbers, the organism begins to 

 multiply rapidly and there is a steadily increasing number of 

 organisms in the blood-stream until eventually the host dies 

 as a result of their activities. In the intermediate case of a 



