352 BIOLOGY: GENERAL AND MEDICAL 



the bacillus of tetanus which, though unable to invade 

 the body generally, eliminates a soluble toxin that usually 

 causes the death of the host by its exciting action upon 

 the nervous system. 



In order that infection be possible, certain conditions 

 must be fulfilled. These, which may be described as the 

 cardinal conditions of infection are: 1. The infecting 

 organism must enter the host in sufficient number; 2. 

 It must enter by an appropriate avenue; 3. It must be 

 virulent; 4. The host must be receptive. 



Among such delicately organized bodies as the micro- 

 parasites the tenure of life is short, and any radical change 

 is apt to be accompanied by a high mortality. When 

 we endeavor to determine the actual number of micro- 

 organisms, in any culture, requisite to infect, we usually 

 find that not a few of the counted and estimated organ- 

 isms are already dead. Infectivity is, moreover, a 

 quality not possessed by all the organisms in equal de- 

 gree. Some are more infective than others, so that it is 

 usually necessary for a considerable number to enter the 

 host in order that sufficiently virulent organisms may 

 survive the change and effect the invasion. 



The number of organisms naturally bears some relation 

 to their virulence or injurious power. If they are of 

 slight virulence a much greater number may be required 

 than when they are highly virulent. Virulence is a 

 variable quality, comparable to the color or perfume of 

 flowers, and, like them, subject to modification through 

 circumstance. It is not known whether mild virulence 

 signifies that all of the organisms in a culture are uni- 

 formly weak in this particular or that many or most of 

 them are. Presumably it is the latter, for when the 

 culture is experimentally placed under conditions 

 favorable to virulence, it sometimes speedily revives. 

 Thus, if bacteria are transplanted many times upon 

 artificial culture media they lose, but if they are passed 

 through a succession of animals for which they are 

 invasive, they increase in virulence. This makes it 



