240 BIOLOGY OF SPECIALIZED GROUPS 



infectious substances coming in contact with water, may be 

 carried by normal persons who have had an attack of typhoid 

 fever at some time previous. Cases are on record where B. 

 typhosus has been carried for eighteen years after an infection by 

 a domestic and then transmitted by personal uncleanliness to 

 several members of the family in which she was employed. 

 B. typhosus is frequently observed in the gall bladder in per- 

 sons having had the disease for long periods. Flies may also 

 carry B. typhosus from feces to various food products, and 

 other individuals may then be infected. 



In bubonic plague the disease is transmitted among the 

 rats, which are very easily infected with B. pestis by means 

 of the bite of the rat flea. A few observations are on record 

 showing that these rat fleas may also bite and infect man. 

 Frequently the flea deposits the bacteria on the skin and by 

 scratching the organisms are inoculated. Some disease-pro- 

 ducing bacteria are largely water-borne, such as B. typhosus. 

 The same is also true of B. dysenteriae and the Msp. comma 

 of Asiatic cholera. 



In diseases like malaria, yellow fever, and some forms of 

 filariasis mosquitoes act as the carriers of the infection. In 

 malaria the hematozoan undergoes certain periods of its life 

 cycle in the mosquito. In yellow fever this is probably the 

 case also, but as the etiological organism is not known we 

 cannot formulate definite information in this regard. 



Protozoan diseases, such as Texas fever, African relapsing 

 fever, are carried by means of the ticks, and the infectious 



