196 A MANUAL OF BACTERIOLOGY 



infectious fevers, others seem to contract them on every 

 possible occasion, and to suffer from all the ills that flesh 

 is heir to. These instances show that there is often a 

 natural insusceptibility to infective disease, or a natural 

 immunity, as it is termed. This may be complete or 

 partial, or it may appertain only to a race " racial 

 immunity " ; or, varying in different individuals and at 

 different ages, it constitutes " individual immunity," as 

 in the case of diphtheria and scarlatina, which become 

 more and more rare as age advances. 



Still more striking, perhaps, is the fact that an insus- 

 ceptibility may be acquired after an attack of infective 

 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 probably 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. Immunity is generally 

 not absolute either to infection or intoxication ; that is, 

 infection can usually be induced under certain 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 susceptible 

 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 phases must be con- 

 sidered. 



