l6o HUMAN BIOLOGY 



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 



^/^v*r» V germs of typhoid fever (Fig. 131) and 



^4l (Pw Asiatic cholera multiply in the small 



■ V"^ <f* intestine. In both these diseases the 



[V ^ pw * source of infection is the diarrhoeal dis- 



fig. 130.— bacillus charges from the alimentary canal. Flies 



of Diphtheria. .- ., . £ . c „, 



may carry the germs on their feet irom 



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 *'\v«»y^*' 



from a certain dairy, because the i\^. ^\-^2 



milk cans have been washed in con- ^ liC >"% "-/ A 



taminated water. In caring for a ty- % ^"nJ^C" X* 



phoid patient all suspicious material » • *** V \ N ^~ *T * V 



should be disinfected or burned. ■"*.«. »***■* *• 



Germs of tuberculosis (called con- FlG ^ H "^ A ™ ° F 



sumption 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. 



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 



