Page:George Archdall Reid 1896 The present evolution of man.djvu/240

Against these diseases protective inoculations of attenuated microbes or of their toxins are therefore useful. But owing to the circumstance that the duration of acquired immunity is different in different diseases, from practically life-long immunity in some diseases—e.g. scarlatina and small-pox—to immunity which endures for but a few days—e.g. relapsing fever—the degree of utility that may be hoped for from protective inoculations varies. In small-pox and kindred diseases, in which the duration of immunity is long, it may be most useful; in relapsing fever and kindred diseases, in which the duration of immunity is short, it is much less useful, except, indeed, in the cases of such dangerous diseases as diphtheria, which saturate the host with their toxins slowly, when inoculation by attenuated microbes or their toxins helps the infected individual to tide over a dangerous crisis, and enables his phagocytes by a series of efforts to accomplish a reaction which they may be unable to perform at a single effort. The quickness and completeness with which the immunity is produced, though not the duration of it, seem in direct proportion to the quickness with which the toxins are produced, and to their degree of virulence. To put it in another way: Natural Selection has so dealt with the micro-organisms of some diseases—e.g. small-pox, measles, &c.—that there has been an evolution in them of the power of producing toxins as their chief means of combating the phagocytes; in which case the phagocytes must react quickly or perish; if they do react quickly they find the feeble microbes an easy prey. But as -regards diseases in which the toxins are neither quickly produced nor virulent—e.g. tuberculosis—Natural Selection has so dealt with the pathogenic micro-organisms that there has been an evolution in them of what may be called personal vigour as their chief means of combating the phagocytes. In such diseases the struggle between