This is the work of an enthusiast who feels keen interest in establishing the protozoan origin of certain infectious diseases. The point of view leads inevitably to a somewhat uncritical survey of the evidence. The enormous mass of recent work on diseases known, or presumed to be due to protozoa, is only incompletely represented. We note, for example, that in the chapter on ticks no mention is made of Rickett's important papers on Rocky mountain spotted fever. Misprints of proper names, such as "Stegomeia" (p. 3), "McNeil" (p. 23) and "Levatadi" (p. 65) are not uncommon.