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of the body. Diphtheria is caused by a species (Fig. 130) that grows on the mucous membrane of the throat; this germ produces a powerful toxin. The germs of typhoid fever (Fig. 131) and Asiatic cholera multiply in the small intestine. In both these diseases the source of infection is the diarrhœal discharges from the alimentary canal. Flies may carry the germs on their feet from the discharge to food. Sometimes typhoid fever cases occur throughout a town because the water supply has become contaminated by sewage. Cases may occur only in families that buy milk from a certain dairy, because the milk cans have been washed in contaminated water. In caring for a typhoid patient all suspicious material should be disinfected or burned. Germs of tuberculosis (called consumption if the disease is in the lungs) may float through the air. Recent investigations indicate, however, that infection usually occurs through the alimentary canal, the germs being swallowed, then absorbed and taken to the lungs in the blood or lymph. To prevent a patient from reinfecting himself in new parts of the lungs or elsewhere, he should carefully cleanse his teeth, mouth, and throat (by gargling with formal or lysol) before eating.

Fig. 130.—Bacillus of Diphtheria.

Fig. 131.—Bacillus of Typhoid Fever.

Mosquito Fevers.—Malaria, yellow fever, and probably dengue are transmitted each by a different genus of mosquito (Fig. 132). A mosquito of the malarial genus may bite a patient and suck into its body blood-corpuscles containing spores of the malarial parasite (a protozoan