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.