DISEASES FROM MICRO-ORGANISMS. 127 



years 50,000 people have been immunized in this way, 

 with failure in only i per cent. 



YELLOW FEVER. 



This is an acute infectious disease the cause of 

 which is not known, but it has been proved that the 

 infection may be transmitted by a certain kind of mos- 

 quito called the Stegomyia fasciata. The blood of 

 yellow-fever patients contains the virus for a period of 

 three days during the sickness, and as the stegomyia 

 feeds on the blood of the patient during this time, it 

 becomes infected. The mosquito cannot transmit the 

 infection at once, not until twelve days have elapsed. 

 If it bites healthy people now, it infects them and the 

 fever develops after an incubation period of five days. 



Yellow fever is primarily a disease of the tropical 

 climate, particularly of the Spanish-American coun- 

 tries. It is occasionally imported to the temperate 

 climate, as numerous epidemics in the seaport cities of 

 the United States testify. To prevent the spread of 

 the disease efforts must be directed to the destruction 

 of the breeding places of the mosquitoes, and to pre- 

 vent them from biting yellow-fever patients. The Preven- 

 former means a complete cleaning up and draining of 

 the swamps and marshes. All yellow-fever patients 

 must be screened to prevent the mosquitoes from biting 

 them.. In countries where the infection prevails, all 

 houses should be screened. Such measures as these 

 rendered the Panama Canal Zone, formerly a hotbed 

 of yellow fever, a safe place in which to live. 



