immune under any circumstances; others are ordinarily immune, but are liable to be attacked when from any cause their vitality is lowered, when their phagocytes are less fit for the battle; others again seem quite unable, even under the most favourable circumstances, to resist infection; their phagocytes succumb easily, and in them the disease usually runs a rapid course, and systemic death quickly ensues.
The fact that, in such a country as England, where tuberculosis has been prevalent for unnumbered generations, so many people are inherently immune, is obviously related to the fact that immunity cannot be individually acquired against it, for, unlike the case of such a disease as measles, against which immunity may be acquired, inborn immunity is here pushed to its extreme by Natural Selection as the sole means to survival in the presence of the disease. It appears, therefore, that in the case of diseases the micro-organisms of which do not paralyze the phagocytes with powerful toxins, but engage from the outset in a physical struggle with them, that immunity cannot be individually acquired, and, therefore, that protective inoculation is useless; but in the case of diseases, the less hardy microbes of which, engaging at long range, produce powerful toxins by means of which they paralyze and keep at a distance the phagocytes, immunity may be acquired, if under normal circumstances the reaction, whereby the phagocytes acquire the power of disregarding the toxins, and so of attacking the microbes, is not too great to be accomplished at one effort; and even when, under normal circumstances, the reaction is too great to be accomplished at one effort (as in rabies), it may be brought about by one or more protective inoculations, by means of which the phagocytes are enabled to accomplish in a shorter or longer series of efforts that which they are unable to accomplish at a single effort.
