Until very recently the bacteria or plant organisms have been given a preponderating and almost exclusive rôle in the production of infectious diseases. The studies in the past few years, however, have brought to light another group of organisms which play an exceedingly important part in the causation of diseases peculiar to the warm countries. Unicellular forms of animal life are to-day the recognized causes of a large number of diseases, whereas but a few years ago they claimed but scanty attention. The pathogenic protozoa in a remarkably short time have risen from an obscure to a commanding position by the side of the pathogenic bacteria.
Under the head of Protozoa are classed: First, the Trypanosomata, which are met with free in the blood plasma; second, the Hemocytozoa, which find their habitat within the blood cells and are represented by the malarial organisms in man and by related forms in