COMMUNICABLE DISEASES 387 



normal serum and the other found in immune serum. Neither substance 

 alone can destroy the cholera bacilli but the two acting together are 

 strongly bacteriolytic. The immunity produced by the Haffkine and 

 Kolle bacterins is temporary only. 



J. Yellow Fever. This highly infectious, but in no wise contagious 

 disease, is peculiar to tropical and subtropical countries. The primary 

 cause is as yet unknown but it is supposed to be a protozoan. The sole 

 carrier of the infection is a mosquito, Aedes calopus. The disease has 

 been highly epidemical in the southern states but since the discovery of the 

 part played by the mosquito the mortality rate has been lowered to a 

 marked degree. In fact the disease is now under complete control. No 

 Aedes mosquitos, no yellow fever. 



It has been observed for a long tune that a frost checked the disease at 

 once, which as is now known, was due to the fact that the frost killed the 

 carriers of the infection. In a general way the statements made under 

 malaria prophylaxis also apply here. Caucasians, especially those not 

 acclimated in the yellow-fever countries, are very susceptible to the disease; 

 Negroes and Latin races are far less- susceptible. 



The history of the establishment of complete yellow fever control is of 

 intense interest. As early as 1881 Dr. Finlay of Havana suggested that 

 the mosquito was responsible for yellow fever, based upon the obser- 

 vation that the disease disappeared as soon as the colder weather killed 

 the animal. In 1900 the United States appointed the Yellow Fever Com- 

 mission placed under the direction of Dr. Walter Reed, with James 

 Caroll, Jesse W. Lazear and Aristides Agramonte as associates. Of 

 these Lazear died of yellow fever and Caroll took the disease but recovered. 

 Dr. Reed has since also died, of spotted fever, which he contracted while 

 investigating this fatal cattle disease. The commission was stationed at 

 Cuba (Havana) and the outcome of their investigation may be sum- 

 marized as follows. 



1 . The specific primary cause was not found, but it was supposed to be 

 an organism similar to that which causes malaria. 



2. The fever is transmitted by the bite of a mosquito. 



3. The mosquito must harbor the infecting agent about 12 days 

 before it becomes actively transmissible to man. The earlier bites (1-8 

 and 10 days) are harmless. 



4. The mosquito Aedes calopus is the intermediary host of the primary 

 cause of yellow fever, while man is the definitive host. 



5. Yellow fever has an incubation period of from 41 hours to 5 days. 



6. Yellow fever is not carried or spread by fomites. The use of dis- 

 infectants is of no avail against the spreading of yellow fever. 



7. One attack of yellow fever establishes immunity to subsequent 

 attacks. 



