240 A MANUAL OF BACTERIOLOGY 



disease or be conferred in certain instances by inoculation. 

 Thus second attacks of smallpox and scarlatina are rare, 

 inoculated smallpox and vaccinia protect against variola, 

 and bacterial vaccines confer considerable protection. 



With regard to the immunity of native races to certain 

 diseases, this is partly due to natural selection and 

 heredity ; during long periods of time, the individuals 

 being all exposed to the same risks, the susceptible ones 

 are weeded out, while the survivors transmit their insus- 

 ceptibility to their descendants ; but this, of course, does 

 not explain the reason for the relatively greater immunity 

 of the insusceptible individuals. In some instances 

 immunity of the adult is due to recovery from an infection 

 in childhood ; this is frequently the case with malaria 

 among native races. Immunity is generally not absolute 

 either to infection or to intoxication ; that is, suscepti- 

 bility may be present under particular conditions. Thus 

 fowls, which are highly refractory to tetanus and tolerate 

 considerable doses of tetanus toxin with impunity, can 

 be tetanised with large doses of an active toxin ; white 

 rats, which are insusceptible to anthrax, become suscep- 

 tible after fatigue, or when fed on an exclusively vegetable 

 diet. Immunity is therefore either (1) natural, or 

 (2) acquired, and it is evinced against either (a) toxins, 

 or (6) micro-organisms, and these different classes must 

 be considered. 



1. Natural immunity against toxins. Toxins cannot 

 enter the body through the intact skin, and frequently 

 not through intact mucous membranes ; hence many toxins 

 may be swallowed with impunity. If the toxin does get 

 into the blood or tissues, there are various non-specific 

 reactions in the body by which it may be eliminated or 

 destroyed. Thus the dilatation of the vessels and the 

 acceleration of the blood-stream which take place in an 

 inflamed area dilute and eliminate the toxin, and the 



