These two simple observations served as foundation and supports to the bridge by which Metchnikoff connected normal biology with pathological biology. Having entered the domain of the latter, he studied various microbian diseases, and asked himself why the organism was sometimes liable and sometimes refractory. In order to elucidate this question, he turned again to lower animals, in which he could easily observe the most intimate phenomena, simplified. He ascertained that liability in an animal corresponded with the fact that microbes introduced into the organism remained free and invaded it, whilst immunity coincided with the inclusion and digestion of the microbes by phagocytes. He also found that, in artificial immunity, the phagocytes are accustomed gradually, by preventive inoculations, to digest microbes and their toxins. Thus he established the fact that phagocytosis and inflammation are curative means employed by the organism. All his ulterior researches, his studies on the various categories of phagocytes and their properties, on their digestive liquids, on the formation of antitoxins, on the different properties acquired by the blood, etc., were but the natural development of those premises. He had proved that the part played by the phagocytes consists, not only in the struggle against microbes and their poisons, but also in the destruction of all the mortified or enfeebled cells of the organism, and that atrophies are nothing more than the absorption of cellular elements by the phagocytes. He found that senile atrophies have the same cause, and asked why the cells of old people's organisms should become enfeebled. He demonstrated that the principal cause is the chronic poisoning of the cells by toxins manufactured by microbes in the intestine. Premature senility was the result--a phenomenon as pathological as any disease. The source of the evil, therefore, resides in the intestinal flora. Accordingly he started to study the latter, as also senility, in order to find means of struggling against both. His researches enabled him to indicate a series of means, based, on the one hand, on the struggle against microbes, and, on the other, on the defence of the noble cells against destructive ones.[38] [38] Replacement of the wild and noxious flora of the intestines by antagonistic cultivated microbes; strengthening and vaccinating of noble cells. The study of old age led him to that of syphilis, a disease which provokes an arterio-sclerosis which is similar to that of old people; the study of the normal intestinal flora was followed by that of intestinal diseases, such as typhoid fever and infantile cholera. Finally, he progressed towards the last phenomenon, the most mysterious in nature, Death. Researches on the silk-worm moth--a rare example of an animal the life of which ends in natural death--allowed him to conclude that the latter is due to an auto-intoxication of the organism.