The value of such protective inoculations is demonstrated in the treatment against small-pox (Jenner), cholera, plague (Haffkine) and typhoid (Wright and Semple).
Simpson, in The Practitioner (Dec. 1906), gives an analysis of the results of Haffkine's serum inoculations as follows: - In Poona, out of 5595 uninoculated cases the incidence was 6.8%, while in 1300 inoculated cases it was only 0.33%.
More remarkable still, animals inoculated with such " attenuated " bacilli proved to be curiously resistant to the deadly effects of subsequent inoculations of the non-attenuated form.
Here manifestly the dose may be easily controlled, and may be gradually increased in successive inoculations.
In the development of toxin-immunity the doses, small at first, are gradually increased in successive inoculations; or, as in the case of very active toxins, the initial injections are made with toxin modified by heat or by the addition of various chemical substances.