The _debility of the system or of individual tissues attacked_, lowers the resistant power, and especially that of leucocytosis, and thus favors survival and encrease of the microbes and their chemical products. Thus the _shock_ succeeding a serious operation, the general depression attendant upon severe illness, or the poisoning by narcotizing ptomaines and toxins, may easily become the extra weight which causes the system to succumb. On the contrary, pre-existing or long standing disease, with consequent general debility appears at times to prove to some extent a protective factor, the previous exposure to the invading germ having educated the leucocytes to resist the toxins and to produce the defensive sera which neutralize the latter or keep the invading microbes in check. A measure of immunization has been secured. The resulting immunization cannot be looked on as very perfect, nor permanent, as a specially strong inoculation by a virulent microbe, or large dose, or different conditions of life, will entirely overcome it, and the pyæmic fever appears. Yet in chronic cases of secondary abscesses from a deep source of infection, the resistance is often such as to ward off febrile pyæmia. In a horse with primary abscess situated deeply under the humerus, free evacuation and healing of the wound, have, in my experience, been followed by the formation of abscesses in distant points for a period of seven years, but without any marked febrile reaction. The _complex nature of the infection_ appears at times to overcome the vital resistance more effectively than will the presence of even a potent microbe alone. Some of the worst cases follow on a wound, the seat of complex infection, and even saprophytes are to be dreaded in this connection. This may operate in various ways, either by mutual combinations or decompositions of the toxic products of associated germs producing more deleterious products, or by the individual action of one ptomain or toxin on leucocytes, hæmatoblasts, sera, or tissue, laying it more open to the attacks of those of another microbe which by itself would have been comparatively harmless. Koch’s experiments showed that the _attack is violent in ratio with the size of the dose_: one-thousandth part of a drop of pyæmic blood was harmless to the rabbit, while one tenth of a drop killed in one hundred and twenty-five hours, and a syringe full in forty hours.