182 OTHER SPOROZOA 



The Yellow Fever Group 



More or less widely distributed in all warm countries are three 

 diseases, yellow fever, dengue and phlebotomus fever, which 

 have many features in common and in fact are often mistaken for 

 each other. They form a series, in the order named, of succes- 

 sively milder diseases. Yellow fever is one of the most vicious 

 of human diseases and is accompanied by a very high fatality; 

 it has been a source of terror in all countries in which it has 

 flourished. Dengue is a much milder disease and is seldom 

 fatal, though its after-effects linger for many months. Phle- 

 botomus or three-days' fever is a still milder disease and, as its 

 name implies, of short duration. Like dengue it has lingering 

 after-effects but they are not so severe or so persistent as in the 

 former disease. 



All three of these diseases are caused by ultra-microscopic 

 blood parasites which are transmitted by dipterous insects in 

 which they apparently undergo part of their life cycle. The 

 diseases each begin with a sudden high fever and headache, and 

 pass through essentially similar stages, intense rheumatism-like 

 aches being very characteristic of all. Each disease confers 

 immunity, almost always permanent in yellow fever, often of 

 long duration in dengue, and very transitory in phlebotomus 

 fever. There seems to be little room to doubt that the parasites 

 of these three diseases must be closely allied, and the possibility 

 exists that immunity conferred by one may give at least partial 

 immunity to the others. Were this found to be the case the in- 

 oculation of the milder diseases, dengue or phlebotomus fever, 

 in regions haunted by yellow fever might be a proceeding well 

 worth the cost. 



Yellow Fever 



Distribution. Yellow fever is a disease which is especially 

 characteristic of the seaport towns of tropical America, although 

 it is also endemic on the west coast of Africa, whence many 

 think it was imported to America with the slaves. Its approxi- 

 mate distribution is shown in Fig. 57. In the past, before the 

 days of the strict quarantine laws now enforced, serious epidemics 

 of this dread disease appeared during the summer in numerous 

 seaports of subtropical and temperate countries. At one time 



