When he returned home, Metchnikoff immediately resumed his work. He continued, with his collaborators, researches on the normal intestinal flora and on the microbian poisons which provoke arterio-sclerosis.

They were able to ascertain that certain microbes of the intestinal flora, such as the bacillus coli and Welch’s bacillus, produce poisons (phenol and indol) which are reabsorbed by the normal intestinal walls and which provoke arterio-sclerosis and other lesions of the organs. A part of those poisons is eliminated by the urine, and the quantity found therein allows one to estimate the quantity contained in the organism. An exclusively vegetarian or carnivorous diet increases its production, while a mixed diet reduces it. During the rest of his life Metchnikoff made systematic and periodical analysis of his own urine in correlation with his diet.

From certain facts and certain experiments he concluded that the reciprocal influence of microbes might be utilised to attenuate or to eliminate the noxious action of some of them. Thus, by cultivating the lactic bacillus in the presence of those microbes which produce poisons belonging to the aromatic group, the decrease in quantity and even the disappearance of phenol and indol is observed. All those facts confirmed anterior results which Metchnikoff had obtained, and indicated the route to be followed in his struggle against those toxins which gradually poison the organism and induce premature senility.

Having thus elucidated certain questions concerning the part played by microbes in a normal organism, he studied the pathogenic intestinal flora. He began by infantile cholera because this question is simplified by the fact that new-born children are fed exclusively on milk. It was then believed by practitioners that this intestinal disease of infants came from their mode of feeding, from summer heat, and other external influences. Metchnikoff, however, succeeded in demonstrating that the contents of the intestines of infants suffering from “cholera” always included a special kind of microbe, the B. proteus; he was also able to give the disease to young anthropoid apes by making them ingest food soiled by the intestinal contents of sick infants, thus establishing the infectious character of infantile cholera.

He then attacked another intestinal disease, typhoid fever, of which the microbe (Eberth’s bacillus) had been known for some time, but had not been studied experimentally, ordinary laboratory animals being refractory. Metchnikoff had again recourse to anthropoids, and succeeded in infecting a chimpanzee by making him eat food soiled by the intestinal contents of a typhoid patient.

With the collaboration of Dr. Besredka, he undertook a series of experiments on anthropoid apes and on macaques. The former alone took typical typhoid fever, similar to that of man. It could be given them by pure cultures of Eberth’s bacillus, which definitely confirmed the specificity of that microbe.

Antityphoid vaccination by means of killed bacilli not being at that time either safe or durable, Metchnikoff advised measures of simple preventive hygiene: the use of cooked food, great personal cleanliness, cleanliness of streets and dwellings, and the destruction of insects, especially flies, which often infect food. In order to popularise these notions, he wrote a series of articles in newspapers. Later, several scientists found efficacious means of vaccination against typhoid fever.

In 1912 Metchnikoff, in collaboration with Dr. Besredka (the author of the antityphoid vaccination method by means of sensitised bacilli), demonstrated on anthropoid apes that antityphoid vaccination by living sensitised microbes is certain, and that it presents no danger of diffusing the disease, for these microbes, harmless to the vaccinated individual, cannot prove a source of danger for his entourage, since they are phagocyted at the very place where they are inoculated.

Metchnikoff always considered that it was very useful to keep the public at large informed of the results acquired by Science, because “it is only by becoming a part of daily life that measures of hygiene and prophylaxis will have efficacious results.” He therefore lost no opportunity of spreading scientific principles and facts. In 1908 he had given in Berlin a lecture on “The Curative Forces of the Organism.” In a Russian review, the Messenger of Europe, he developed the same subject and included an epitome of his lecture in Stockholm on immunity. In that article he expounded the phagocyte theory of immunity. Among concrete examples of its application, he quoted the indications concerning the evolution of an infectious disease provided by the quantity of leucocytes in the blood, and the process employed by certain surgeons to diminish the danger of infection during an operation: just as, in case of an enemy menace, the Government mobilise an army, certain surgeons employ divers means to attract an army of phagocytes and to stimulate their activity in case any microbes should penetrate into the wound.

In 1909 he gave another lecture at Stuttgart, “A Conception of Nature and of Medical Science,” in which he summed up his two works Études sur la nature humaine and Essais optimistes. The title of this lecture was intended to emphasise his view of human nature, according to which “Man, as he appeared on the earth, is an animal and pathological being belonging to the realm of medicine.” But he ended his paper by the same optimistic thought which illumines the whole philosophy of his later years. “With the help of Science, Man can correct the imperfections of his nature.”

He unveiled these imperfections and the ills which proceed from them, not only from a love of truth or scientific honesty, but always with the object of finding means to combat them. He never allowed sight to be lost of the fact that Science lights up the tortuous and painful path which leads to an issue that suffering humanity will find by gradually widening the limits of knowledge with the help of Work and of Will.

Thus all his writings offer us encouragement and support.