BY WILTON W. R. LOVE, M.B. 125 



virulence, and that when subcultures were made the diminished 

 virulence persisted. Such cultures can be used for protective 

 inoculation, and are generally known as vaccines, from their 

 analogy to the action on the human organism of the material 

 derived from cow-pox. Again, an organism may be "attenuated," 

 that is, reduced in virulence by passing through another animal, 

 e.g. Burdon Sanderson and Greenfield shewed that anthrax 

 bacilli when inoculated into guinea pigs became attenuated, and 

 could then be used for protective inoculation of sheep and cattle ; 

 c.p., also Pasteur's experiments with swine-plague, where rab- 

 bits were used to attenuate the bacilli. Similarly some organ- 

 isms become diminished in virulence if grown at an abnormally 

 high temperature, or in the presence of weak antiseptics. 

 Exaltation of the virulence on the other hand, may be brought 

 about chiefly by the method of cultivating the organism from 

 animal to animal — the method of " passage" discovered by Pasteur 

 — the animals used are mostly rabbits or guinea-pigs. This 

 method can be applied to the organisms of typhoid, cholera, 

 pneumonia, to streptococci, and staphylococci. Similarly the 

 above methods may be combined, e.g., by injections of cultures at 

 first attenuated and afterwards more virulent, and by increasing 

 the doses a high degree of immunity may be gained. Haflfkine's 

 anti-choleraic injection dependsuponthiscombination— the virus 

 {i.e., the cholera organisms) is first attenuated by passing a 

 current of sterile air over the organisms, which are then passed 

 through guinea pigs by injection into the peritoneum. The 

 virulence is thus increased 20-fold, i.e., ^V}th of the ordinary 

 lethal dose of the culture is sufficient to kill. Animals are first 

 treated with the attenuated virus, and then gradually with the 

 *' exalted " virus. This process has been tried by Haffkine on 

 the human subject with marked success. Three injections of 

 attenuated virus are first given, and then virus e.valte is used. 



(b) Inoculation with small and repexted doses of fully 

 virulent organisms. The animal tissues can deal with a small 

 dose of virulent organisms ; this produces a small amount of 

 immvxnity in the animal, which is taken advantage of to intro- 

 duce a larger dose, and so on, but up to a certain point only — 

 for any animal, however highly immunised, can be killed by a 

 sufficiently large dose ; in other words, any individual, provided 

 the species is susceptible to a given disease, may succumb to an 

 attack of that disease if the dose be sufficiently large and 

 virulent. 



