Malarial fever prevails especially where heat, moisture and decomposing vegetation are present because these are favorable to the existence of mosquitoes. Malarial infection occurs most commonly after sunset, because mosquitoes, after remaining in hiding during the day, set out upon their depredations after dark. The infection is not widely diffused in a horizontal, oblique or vertical direction because mosquitoes do not fly far from their place of origin, and especially not to a great height. The infection is not conveyed, but rather hindered, by winds, because the insects do not leave their hiding-places when the wind blows. Woods, instead of preventing, may be foci of malarial infection, because the mosquitoes make their hiding places in damp woods and trees. A great advance was made when, in 1880, Laveran, a French army surgeon at Algiers, discovered the parasite of malarial fever, but it was not till fifteen years later that their