160 HUMAN BIOLOGY 



of the body. Diphtheria is caused by a species (Fig. 130) 



that grows on the mucous membrane of the throat; this 



^ *t% germ produces a powerful toxin. The 



*^Y^]5& & erms of typhoid fever (Fig. 131) and 



~{(0 f r (fi Asiatic cholera multiply in the small 



1 * "S^S? intestine. In both these diseases the 



/ 1 ^QJL 



i source of infection is the diarrhceal dis- 



FIG. 130. BACILLUS charges from the alimentary canal. Flies 

 OF DIPHTHERIA. 



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 * 'N^^ 

 from a certain dairy, because the , \ ^ v/ 

 milk cans have been washed in con- ^ I[C "*V\ 

 taminated water. In caring for a ty- % ^"vJ^* \. * 

 phoid patient all suspicious material ^ \\ v ^~ \~ / \ 

 should be disinfected or burned. "'tJ*^*^ x 



Germs of tuberculosis (called con- FlG - 131- -BACILLUS OF 



TYPHOID FEVER. 

 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 



