YELLOW FEVER 71 



form a zigzag line by the alternate right angle change of direction 

 of each consecutive spiral. The organism is smaller than that 

 of infectious jaundice and is actively motile. 



Since the mosquitoes cannot transmit the disease by biting 

 until 12 or 14 days after sucking infected blood the parasites 

 evidently undergo a cycle of development in the mosquito. The 

 appearance and habits of the yellow fever mosquito are described 

 on page 443. 



The Disease. Yellow fever has an incubation period of from 

 three to six days. The first symptoms are severe headache and 

 aches in the bones, followed by a sudden fever during which 

 the face is flushed and swollen and the skin dry. This fever 

 slowly subsides, and after three or four days there is a period 

 of " calm " during which the temperature is near normal but the 

 pulse very slow. By the third day the skin usually becomes a 

 characteristic yellow color, which, as the disease progresses, 

 changes to a deep coffee brown. A striking but not invariable 

 symptom, and one of ill omen, is the " black vomit," a gushing 

 up through the oesophagus of a coffee-colored or even black fluid, 

 consisting largely of fragments of red blood corpuscles and freed 

 haemoglobin, and sometimes even pure blood. The period of 

 " calm " may lead to recovery in a few days or there may be a 

 second fever which lasts irregularly for a longer time than the first. 



Yellow fever is a very fatal disease. During the French oper- 

 ations at Panama relay after relay of laborers were stricken 

 with the yellow plague and were turned loose to die without mercy 

 or help, to be replaced by a new set. Not only the laborers but 

 the engineers, nurses and others were stricken down. One vessel 

 is reported to have brought over 18 young French engineers, 

 all but one of whom died of yellow fever within a month after 

 their arrival. 



Fortunately yellow fever gives a permanent immunity after 

 one attack has been successfully withstood. In children the 

 disease is often very mild so that it is frequently not even recog- 

 nized, yet the immunity it gives is permanent. Natural im- 

 munity is unknown in any race, sex or age, though the negroes 

 suffer less from the disease and have a much lower per cent of 

 mortality than the whites. 



Treatment and Prevention. Until the discovery of the or- 

 ganism causing yellow fever by Noguchi and its isolation in pure 



