IMMUNITY AND SUSCEPTIBILITY 113 



1. The age of the animal, young animals being often 

 affected by injections that would not affect full-grown 

 animals of the same species. 



2. The condition of the animal's health. When in 

 robust health, infection in whatever way presented is not so 

 easily taken as when the system is debilitated ; for example, 

 an attack of typhoid often occurs after exposure to sewer 

 gas, which does not itself contain the organism, but, by 

 lowering the general tone, causes the virus, which might 

 otherwise have been inactive, to take effect. 



3. The manner in which infection is presented, whether 

 aerially, or in food, or by traumatic inoculation, or by 

 contagion. 



4. Lastly, in the case of infection by artificial cultures, 

 the age of the culture and the medium on which it has 

 been grown. 



As an example of the last case, it is well known that a 

 culture of the tubercle bacillus, which has been subcultured 

 through many generations on artificial media, will lose its 

 virulence or infective power to a considerable extent, so that 

 a much larger dose will be required to produce an effect on 

 a susceptible animal, but that by passing the organism 

 through an animal its virulence may be restored. 



The virulence of an organism may be decreased, or 

 ' attenuated,' artificially ; for example, by exposing cultures 

 of anthrax to a temperature of 40° C. for some time, they 

 become attenuated to such a degree that they will kill 

 nothing larger than mice. 



The effect on animals of an organism may be greatly 

 ■enhanced by the injection along with it of some other 

 organism that has not pathogenic properties, but which in 

 some way that we do not yet understand adds greatly to 

 the virulence of the pathogenic organism which it accom- 

 panies. 



