INFECTION AND IMMUNITY. 31 



infection depends in part on their power of producing on the 



. part of 



disease, that is, their virulence. Conditions that are bacteria 

 not suited to the growth of bacteria will diminish or 

 destroy the virulence ; the continued cultivation of bac- Viru _ 

 teria outside the body on artificial culture media will 

 do this. Bacteria that have lost the power of pn>- 

 ducing disease are spoken of as being attenuated. 

 Another factor that modifies infection is the number At ^a- 

 of bacteria that invade the tissues. While the exact 

 number of bacteria necessary to cause infection is not 

 known, it may be said that the greater the virulence 

 the fewer the bacteria required. The path by which 

 bacteria enter the tissues frequently determines 

 whether infection is caused or not. The bacilli of 

 typhoid fever to cause infection must be swallowed, 

 but if they are rubbed into the skin no infection results. 

 On the other hand, the pus-forming bacteria like the AV infe?- 

 staphylococci and streptococci may be swallowed with- 

 out causing infection, but if they are rubbed into the 

 skin a boil or an abscess is almost sure to result. So 

 to cause infection bacteria must enter the body through 

 channels best adapted to their growth and multi- 

 plication. 



Concerning the individual exposed to infection it ^ Tt ila f t 

 is known that everyone is endowed to a variable de- 

 gree with defensive substances in -the blood and 

 tissues that tend to overcome and destroy invading 

 bacteria. Unhealthy people, as everyone knows, are 

 more likely to become infected and to succumb to in- 

 fection than the healthy. This power of the human 



