This book reviews the development and present state of knowledge concerning the human diseases that it accepts as caused by ultramicroscopic viruses. It opens with an instructive and comprehensive discussion of ultraviruses by Levaditi followed by a section on their nature by Gratia. Vaccinia, variola and vaccination are discussed by Gastinel, biologic purification of vaccinial virus by Mesrobeanu, postvaccinial encephalitis and postinfectious encephalitis in general by J. P. Bijl. Nicolau reviews herpes, Mohlwill zona (herpes zoster), Lépine rabies and its virus (104 pages), Levaditi epidemic encephalitis and epidemic poliomyelitis, with a chapter on immunity in poliomyelitis by Hornus; Lépine, typhus fever and similar fevers; Mathis, yellow fever; Blanc, dengue; Vieuchange, psittacosis; the Levaditis, inguinal lymphogranulomatosis (maladie de Nicolas-Favre); Kreis, lymphocytic choriomeningitis (Armstrong's disease); Haber, influenza. There is a section on filtration by Lecomte du Nouy, on ultrafiltration and its applications to the study of viruses by I. A. Galloway, on