*Fumigation.*—To destroy germs in the air or adhering to the walls of rooms, furniture, clothing, etc., fumigation is employed. This is accomplished by saturating the air of rooms with some vapor or gas which will destroy the germs. Fumigation is quite generally employed in the general cleaning after the patient leaves his room. This, to be effective, must be thorough. Formaldehyde is considered the best disinfectant for this purpose, and it should be evaporated with heat in the proportion of one half pint of the 40 per cent solution to 1000 cu. ft. of space. Since formaldehyde is inflammable and easily boils over, it has to be evaporated with care. It should be boiled in a tall vessel (a tin or copper vessel which holds about four times the quantity to be evaporated) over a quick fire, the room being tightly closed (openings around windows and doors plugged with cotton or cloth). After three or four hours the room may be opened and thoroughly aired. Since formaldehyde is most disagreeable to breathe, one should not attempt to occupy the room until it is free from the gas. This will require a day or more of thorough ventilation. *Facts Relating to the Spread of Certain Diseases.*—The problem of preventing disease in general often resolves itself into the problem of preventing the spread of some particular disease. It is then of vital importance to know the special method by which the germs of this disease leave the body of the patient and are conveyed to the bodies of others. Some of these methods are novel in the extreme, and are not at all in accord with prevailing notions. Particularly is this true of that disease known as *Malaria, or Malarial Fever.*—This disease, so common in warm climates and also prevalent to a large extent in the temperate zones, is due to animal germs (protozoa), which attack and destroy the red corpuscles of the blood. These germs, it is found, pass from malarial patients to others through the agency of a variety of mosquitoes known as _Anopheles_. In sucking the blood of a malarial patient, the mosquito first infects her own body.(131) In the body of the mosquito the germs undergo an essential stage of their development, after which they are injected beneath the skin of whomsoever the mosquito feeds upon. For the spreading of malaria, then, two conditions are necessary: first, there must be people who have the disease; and second, there must be in the neighborhood the special variety of mosquito that spreads the disease. If either condition be lacking, the disease is not spread. The malarial mosquito (_Anopheles_) may be distinguished from the harmless variety (_Culex_) by the position which it assumes in resting, as shown in Fig. 170. [Fig. 170] Fig. 170—*Mosquitoes* in resting position. (From Howard’s _Mosquitoes_.) On left the malarial mosquito (_Anopheles_); on the right the harmless mosquito (_Culex_).