In 1900, Metchnikoff presented to the International Congress in Paris a complete tabulation of his researches and fought his contradictors for the last time, after which, convinced that his deductions were solid, he began to write a work on _Immunity in Infectious Diseases_. In it he epitomised, as in a great harmonious chord, the results of his researches, reaching over a period of nearly twenty years; he affirmed and gave final expression to his doctrine of immunity, based on the comparative study of the mechanism of that phenomenon and of its evolution along the whole scale of living beings; he related his controversies, analysed the objections to his doctrine, expounded the theories of other scientists concerning immunity, and gave a general view of the present state of the question. This book is a living picture of a long and important part of Metchnikoff's scientific achievements. * * * * * The question of immunity is of such great importance, the mechanism of this phenomenon and the physiology of intracellular digestion are so complicated, that I have thought it useful to epitomise here the exposition given of it by Metchnikoff in his book. Readers who do not care to go further into the subject can pass over the next few pages without hindering their comprehension of the following chapters. Diseases affect all living beings, and the greater number of plants and animals would cease to exist without innate or acquired immunity. Unicellular beings are generally immune against infectious diseases, which are rarely observed in them. Their body being almost entirely made up of digestive protoplasm, the microbes which they absorb are directly introduced into a noxious medium and are destroyed therein like any other food. If the microbes are indigestible, they are immediately rejected; hence, in the majority of cases, they cannot become harmful. This resistance of unicellular beings to many microbes and microbian toxins is due not only to the intense digestive power of the cell but also to the extreme sensitiveness which rules over the choice of food. Owing to this protoplasmic sensitiveness (_chimiotaxis_) protozoa are attracted towards certain microbes or substances (positive chimiotaxis) and repelled by others (negative chimiotaxis). Thus, many ciliate infusoria choose bacteria only for their food; they are sharply repelled by dead infusoria, etc. Therefore, in the _natural_ immunity of unicellular beings, two fundamental elements may already be observed: sensitiveness and intracellular digestion. No researches have yet been made on the possibility of conferring on protozoa an artificial immunity against certain pathogenic microbes and their poisons. But unicellular beings, insensible to microbian poisons, are the reverse to many chemical substances which, in their normal life, they have no opportunity of ingesting.