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VI] although, of course, those engaged in tilling and working the soil are more exposed to mosquito bite, and therefore more subject to malaria, than the townsman or the sailor. Malarial attacks are more severe, more common, and much more dangerous in young children than in adults.

Malaria a commmunicable disease.—— Malaria is certainly not directly communicable, in the same way that such diseases as smallpox or measles are. It can be communicated directly only by injection of malarial blood. But there can now no longer be any doubt that a malarial patient introduced into a community, provided suitable mosquitoes are present, is a source of danger. If mosquitoes of the proper species bite such a patient say in the wards of a hospital, in a gaol, a house, or a camp and a week later bite someone else, that second individual may become infected, and ten days later may be seized with malarial fever.

Study of the mosquito indispensable.——Full knowledge of all that concerns the etiology of the disease will only be attained when we have full knowledge of the various species of mosquito capable of subserving the germ, of certain vertebrates which may be capable of taking the place of man in the malarial cycle, of their geographical distribution, of their habits, and of their enemies. As yet this knowledge is but beginning. When we have said that the alaria parasite is subserved by several species of Anophelinse and that these species are mainly of nocturnal habit, we have enumerated the principal items of existing knowledge on the subject. Whether certain species of Culex, Stegomyia and other Culicinæ are efficient hosts for the parasite, as some of them certainly are for Plasmodium danilewskyi and Filaria bancrofti, can as yet neither be affirmed nor denied. Studies in this field are being actively carried on, and important advances may be looked for.

Personal acclimatization.—— Is there such a thing as acquired immunity as regards malaria? The