organisms, which are, however, destroyed by the phagocytes before they are able to multiply and produce the disease. Thus small doses of toxins are introduced into the system, in response to the stimulation of which the phagocytes react to some extent, and attain a position of some advantage, which, however, since the stimulation is so much less, is not so great or so permanent as that which results in an individual who has actually experienced and recovered from the disease. Such powers are therefore quite lapsed during residence in a lion-infected area, and therefore an individual who returns from a lengthened sojourn in it is liable to fall a victim to the disease to which he was formerly immune.
A third point of interest is the following: diseases in which the toxins are powerful are usually exclusive each one of all others; that is, such diseases seldom, if ever, co-exist in the same individual. The reason for this must be that the toxins of a disease that has established itself, poisonous as they are to the component cells of the host, are equally or more poisonous to the microbes of other diseases, which therefore are unable to multiply in such an unfavourable environment. Probably these low unicellular organisms are not able to vary in a fit direction like the cells of high multicellular organisms, and for that reason probably it is that we sometimes hear of a long-established disease,—e.g. lupus,—the toxins of which are not virulent, being cured or abated after the host has suffered from some other disease, the toxins of which are virulent—e.g. erysipelas. In such a case the toxins of the latter must be even more effective against the microbes of the old-established disease than against the phagocytes of the host; must even more enfeeble them than the phagocytes; in consequence of which, when recovery from the new disease occurs, the old disease is also conquered.
