a result of the experiments we know that a mosquito which 

 has bitten a fever-stricken patient during the first three days 

 of the disease is capable, after an incubation-period of from 

 twelve to eighteen days, of transmitting yellow fever to 

 every non-immune person it bites during the rest of its life. 

 After about the third day of the disease the infective princi- 

 ple disappears from the blood and mosquitoes can no longer 

 become contaminated. 



Just as in the history of malarial fever, other theories have 

 been current regarding the cause of yellow fever. Chief 

 among these was the theory of transmission by f omites {i. e. 

 articles or inanimate objects that have come in contact with 

 yellow fever patients or their immediate surroundings). 

 That the disease can not be conveyed by fomites and that 

 mosquitoes are the onlv agents of infection has been abun- 

 dantly proved by the army experiments in Cuba. A mos- 

 quito-proof hut was erected and filled with clothing, bedding, 

 etc., soiled and contaminated by contact with yellow fever 

 patients. Amid these unwholesome surroundings three non- 

 immune young Americans slept for twenty nights and 

 emerged in perfect health. In another building, thoroughly 

 disinfected, a volunteer developed yellow fever from the bite 

 of an infected mosquito, while other men in the same room 

 but protected from mosquitoes remained in perfect health. 

 The knowledge gained through these experiments proves that 

 disinfection of freight and baggage from yellow fever regions 

 is needless if care be taken to see that it contains no live 

 mosquitoes. 



On the analogy of malaria these facts suggest that there 

 must be a microscopic or ultra-microscopic parasite of Stepo- 

 myia fasciata which, in order to complete its life cycle, must 

 be transferred to the blood of a human being as intermediate 

 host, and that this parasite is the cause of yellow fever. Up 

 to the present time, however, such a parasite has not been 

 isolated although we know the place and duration of both 

 stages of its life history. 



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