Nearly all parts of the temperate and torrid zones are subject to some form of malarial disease. There is probably little doubt in the minds of any great number of intelligent physicians at the present day, that the cause of this class of affections is an organic germ of some kind, although its exact nature may not be as yet precisely made out Malarial diseases occur with the greatest frequency in the vicinity of marshes and lands subject to overflow, as borders of lakes, low lands adjoining rivers, etc. The danger from these sources exist not while the soil is submerged, but while it is drying up. A great increase of frequency in the occurrence of malarial disease has also been observed to result from the breaking up of new land, and especially from the exposure of what is termed "made land," in digging trenches for the purpose of laying water pipes, etc., in cities. In New York City, a large portion of which is built upon low marshy land which has been filled up since the city has been improved, it has been frequently observed that malarial diseases of various sorts quickly make their appearance upon streets in which deep trenches are being dug for the purpose of laying water and gas pipes. It would be a mistake to suppose that low marshy districts are the only ones affected. For some reason not well understood, certain localities which exist at quite high altitudes are also subject to malaria. For example, it is met with in the Apennines at a height of 1,100 feet, in the Pyrenees at an altitude of 5,000 feet, in the island of Ceylon, more than 6,000 feet above the level of the sea, and in Peru, at a height of 10,000 feet It is found upon the high bluffs of Gibralter, as well as on the low plains of Italy.

Not infrequently an individual may be exposed for months or even years to malaria without the appearance of any of the characteristic symptoms of malarial poisoning until the attack is excited by unusual fatigue, taking cold, exposure to fog or night air unusually heavily laden with the poison, or some similar cause. Experience seems to show that exposure to the poison when the stomach is empty, especially early in the morning or evening, is very likely to occasion an attack. Sleeping in damp beds, living in basements or cellars, or in houses densely shaded by tall trees, may render the system susceptible to the poison, and thus occasion an attack. There is also reason for thinking that the disease may be communicated through water. The author of the article on malarial disease in Ziemssen's Encyclopedia, reports a case in which a body of soldiers who filled their canteens from a marshy district before embarking on a voyage, all suffered symptoms of malarial poisoning soon after drinking the water, the only ones of their number who escaped being the few who purchased water from the sailors, none of whom were attacked. The disease may make its appearance in a few hours after a person has been exposed to the poison, as by a ride in the night air, or a boat ride in the evening or early morning; or several weeks or months may elapse before the characteristic symptoms make their appearance.