Under the name of malaria is included a group of morbid symptoms formerly supposed to be due to a miasm or bad air, but now known to be caused by protozoan parasites of the genus _Plasmodium_, which attack the red blood corpuscles. It occurs in paroxysms, each marked by a chill, followed by high fever and sweating. The fever is either intermittent or remittent. There are three principal types of the disease, due to different species of the parasite. They are: 1. The benign-tertian, caused by _Plasmodium vivax_, which undergoes its schizogony or asexual cycle in the blood in forty-eight hours or even less. This type of the disease,--characterized by fever every two days, is the most wide-spread and common. 2. The quartan fever is due to the presence of _Plasmodium malariæ_, which has an asexual cycle of seventy-two hours, and therefore the fever recurs every three days. This type is more prevalent in temperate and sub-tropical regions, but appears to be rare everywhere. 3. The sub-tertian "æstivo-autumnal," or "pernicious" fever is caused by _Plasmodium falciparum_. Schizogony usually occurs in the internal organs, particularly in the spleen, instead of in the peripheral circulation, as is the case of the tertian and quartan forms. The fever produced is of an irregular type and the period of schizogony has not been definitely determined. It is claimed by some that the variations are due to different species of malignant parasites. It is one of the most wide-spread of human diseases, occurring in almost all parts of the world, except in the polar regions and in waterless deserts. It is most prevalent in marshy regions. So commonplace is malaria that it causes little of the dread inspired by most of the epidemic diseases, and yet, as Ross says, it is perhaps the most important of human diseases. Figures regarding its ravages are astounding. Celli estimated that in Italy it caused an average annual mortality of fifteen thousand, representing about two million cases. In India alone, according to Ross (1910) "it has been officially estimated to cause a mean annual death-rate of five per thousand; that is, to kill every year, on the average, one million one hundred and thirty thousand." In the United States it is widespread and though being restricted as the country develops, it still causes enormous losses. During the year 1911, "in Alabama alone there were seventy thousand cases and seven hundred and seventy deaths." The weakening effects of the disease, the invasion of other diseases due to the attacks of malaria, are among the very serious results, but they cannot be estimated.