548 A MANUAL OF BACTERIOLOGY 



non-immune individuals, and precautions were taken to 

 prevent the introduction of yellow fever from outside. 

 Five individuals were bitten by infected mosquitoes, and 

 four out of the five contracted yellow fever, no other 

 occupants of the camp being attacked by the disease. 

 Subsequently several non-immune individuals were exposed 

 to yellow fever infection from soiled linen, yellow-fever 

 discharges, etc., in a mosquito-proof hut from which 

 mosquitoes were excluded, with entirely negative results. 

 These experiments prove, therefore, that yellow fever is 

 conveyed by mosquitoes only, and further work by 

 Americans and Cubans, and by French and Brazilian 

 Commissions, has entirely confirmed these researches 

 and conclusions. It has been found that to convey infec- 

 tion, it is necessary for the mosquitoes to bite the patient 

 during the first three or four days of the illness, but they 

 do not become infective until about the twelfth day after 

 feeding, and then retain their infectivity indefinitely. 

 All these facts point to a protozoon as being the causative 

 organism, but none has been found with certainty. 



The Americans have shown that the blood-serum after 

 filtration through a porcelain filter is still infective ; the 

 organism, therefore, is probably ultra-microscopic, at least 

 at one stage. Seidelin 1 describes extremely small rounded 

 bodies with a minute chromatin point and feebly staining 

 protoplasm, without pigment, in the blood corpuscles. 

 Somewhat similar, but larger, bodies may also be present 

 in the organs and free in the plasma. Macfie and Johnston 2 

 state that they have found elements similar to those 

 described by Seidelin in the red corpuscles in practically 

 every case of yellow fever examined. 



1 Journ. Pathol. and Bacterial, vol. xv, 1911, p. 282. 



2 Proc. Roy. Soc. Med., vii, No. 3, 1914 (Med. Sec.), p. 49. 



