such can be made, to the influence of an altered environment in an artificial medium; but great difficulties will lie in the way of' success by the latter method, since syphilis, so far as we know, is a purely parasitic disease, never a saprophytic one; that is, the microorganisms which produce it are capable of existence in a living host, but not, so far as we know, in non-living nutrient media. The difficulty in the way of success by the former method lies in the fact that we know of no animal capable of taking the disease and attenuating it.
In diseases which have a shorter incubation period than rabies—i.e. in which the toxins are more rapidly produced—but in which the micro-organisms do not pervade the body, but are localized in a definite area of limited extent—e.g. in diphtheria and tetanus—it is only reasonable to assume that the toxins elaborated by them, however much they may paralyze or destroy the phagocytes in the immediate neighbourhood of the local lesion, where they are in a state of concentration, however much in the concentrated state they may ward off the attack of the phagocytes on the microbes, do not at first destroy or paralyze the phagocytes in the blood and tissues at a distance where they are in a state of great dilution; and therefore, as regards these diseases also, we may hopefully attempt to cause recovery and produce acquired immunity by introducing, before the virulent toxins reach such a degree of concentration as to destroy or paralyze the phagocytes, less virulent toxins obtained from animals—e.g. horses—which "attenuate" their microbes, as the microbes of rabies are attenuated in the monkey, and as the microbes of small-pox are attenuated in the calf, or else from artificial cultures, in which the microbes are attenuated, as the microbes of rabies are attenuated, in a slowly drying cord, and as the microbes of anthrax are
