cholera—can not only exist but can multiply, though apparently only to a limited extent, outside it, while the microbes of yet others—e.g. malaria—can not only exist and multiply outside it, but can multiply there to an unlimited extent.
Of the microbes of cholera, and probably also of typhoid, it is interesting to remark, that though it seems they are able to multiply for a period in contaminated fluids, yet in time they always perish, unless they find a home again in a human host, as appears to be proved by the fact that travellers are never infected by them in long-deserted districts. That they are truly parasitic when in man, in the sense that they find their nutriment essentially in his living tissues, not in the non-living contents of his bowel, is proved by the circumstance that he is able to acquire immunity against them; or rather, it is proved by the fact that after he has once acquired immunity the microbes of the disease are no longer able to flourish in the contents of his bowel even should they gain fresh entrance. Reasoning from the analogy of other diseases, acquired immunity in this disease can only be due to the phagocytes acting from a "position of advantage" on the microbes within their reach, which of course would not be those that lay free in the bowel, mixed with its contents; the only other rational theory here being one which supposes that it is due to the secretion, by the cells lining the intestinal tract of an immune person, of a substance poisonous to the microbes; an untenable theory when we take into consideration the fact that immunity may be acquired by individuals whose ancestry can have had no exjierience of the disease, and therefore cannot have developed the power of secreting this particular complex chemical substance, this species of anti-toxin — not only exactly when it was first wanted, i.e. when infection first took place in the individual, but ever after.
