258 CIVIC BIOLOGY 



have malaria-like diseases caused by different species of the genus 

 Piroplasma P. equi, P. ovis, P. cants, etc. ; and birds, frogs, turtles, 

 and many other animals serve as hosts for blood parasites of other 

 kinds. 



Yellow fever. No one has been able to demonstrate the parasite of 

 yellow fever, although many investigators have hunted for it diligently. 

 It is so small that it passes through the pores of a Berkefeld filter and 

 is therefore supposed to be too small to see with a microscope. This may 

 be true, or the organism may be soft and elastic enough to squeeze 

 through the pores of a filter, and so transparent and unstainable that 

 no one could recognize it in the field of the microscope. It is generally 

 agreed that the organism is a protozoan, because it is proved to have 

 a life cycle in a certain species of mosquito (Aedes calopus, formerly 

 named Stegomyia fasciatd) and is transmitted solely by its bite. It 

 has been transmitted experimentally by injecting into nonimmunes 

 a few drops of blood (or the serum of such blood after passing 

 through a Berkefeld filter) drawn from yellow-fever patients during 

 the first three days of the attack. After filling with yellow-fever 

 blood the mosquito is not infective for at least twelve days, indicating 

 a definite life cycle, and then the mosquito remains infective as long 

 as she lives fifty-seven days in one case. (For discussion of this 

 topic in relation to mosquito extermination see Chapter XI. 1 ) Few 

 stories of discovery are more instructive or fuller of inspiration and 

 hope for the future than this work upon the cause and prevention of 

 yellow fever. Will some member of the class volunteer to look it up 

 and report? 



Smallpox. This is clearly a parasitic disease, the germ of which has 

 eluded discovery, as have the organisms that cause measles and scarlet 

 fever epidemic disorders of the same class. We have, however, gained 

 control of it by vaccination. About 1770 Edward Jenner happened to 

 hear a woman say : " I can't take smallpox, because I have had cowpox." 

 The idea was common at the time in several countries. Jenner studied 

 the problem of immunity among the dairymaids for twenty-six years. 

 On May 14, 1796, he made his first experimental taccination, upon 

 James Phipps, son of one of his friends. On July 1 he vaccinated James 

 again with virus from a case of smallpox, at the same time vaccinating a 



1 Sternberg, "The Transmission of Yellow Fever by Mosquitoes," Popu- 

 lar Science Monthly, Vol. LIX (1901), p. 225 ; Kelly, Walter Reed and Yellow 

 Fever, New York, 1907 ; McCaw, Walter Reed Report, Smithsonian Insti- 

 tution, 1905, p. 549. 



