attenuated when subjected to a temperature of from 42° to 43° C.
But in diseases in which the incubation period is short, and in which the microbes pervade the whole body, any attempt to produce immunity by inoculation with a weaker virus, after the host has been infected by the stronger virus of the normal disease, must necessarily be vain. In such a case the phagocytes, paralyzed or destroyed by the stronger toxins, are unable to react in response to the stimulation from the weaker toxins, and death ensues; or if death does not ensue it is because the phagocytes have been able to react from a position of no acquired advantage to the stimulation of the strong toxins. So skin-cells in the presence of great heat disregard and do not react to the stimulation of lesser heat (which then but adds to the effect of the other), or if they do react at all and are not destroyed, react in response to the stimulation of the greater heat. So other kinds of cells in the presence of large doses of opium, tobacco, alcohol, &c. (toxins in fact) disregard the action of smaller doses given shortly after (which indeed but supplement the action of the greater doses), and perish, or if they do not perish, but react, react to the stimulation of the stronger doses. Therefore in such a disease as small-pox, in which the toxins are rapidly produced, and in which the microbes pervade the whole body, protective inoculation after infection is useless. To be effectual it must be made at a time antecedent to infection, which is what is done in vaccination against small-pox; at the least if made after infection it must be made at a time antecedent to the appearance of symptoms of illness— i.e. during the incubation period, before the virulent toxins have been elaborated,—and then it will be successful only if the attenuated microbes elaborate
