30 THE SMALLEST LIVING THINGS 



or four days of the disease. It was further established that once 

 a mosquito was infected the infection was for the lifetime of the 

 mosquito. The ability to transmit yellow fever is practically 

 limited to one species of mosquito, Aedes (Stegomyia) fasciatus, 

 which has a world-wide distribution but breeds only in the tropics 

 and the warmer parts of the temperate zones. 



The entire field of yellow fever prophylaxis became known 

 long before the organism causing the disease was seen or its 

 type known. It was supposed to be due to one of the filterable 

 viruses until the Japanese bacteriologist, Hideyo Noguchi, who 

 lost his life in Africa while studying an African type of yellow 

 fever, demonstrated in 1919 a spirochaete as its cause. This he 

 named Leptospira interrogans, distinguishing it from other 

 spirochaetes by the hook-like turn of the spiral at one end, and 

 associating with it the cause of another human disease, infectious 

 jaundice (Weil's disease), under the name of Leptospira ictero- 

 hemorrhagiae. These two types differ so little from other spiro- 

 chaetes that authorities generally include them with Treponema. 



Similar blood diseases, due to the spirochaete Treponema gal- 

 linarum (Fig. 16), are found in ducks and in fowls generally. 

 Their transmission is brought about from bird to bird by a tick, 

 called Argas miniatis, living in the feathers of the bird. 



It is hardly possible to draw a line between these blood-dwell- 

 ing spirochaetes, which by some authors are called Spirosoma, 

 and the types which invade other tissues or the true treponemas. 

 The blood-dwelling forms of Treponema duttoni in human blood 

 become tissue-invading forms in the tick, and similar combinations 

 of mode of life are characteristic of the majority of pathogenic 

 types. So it is with Treponema pallidum, the cause of syphilis 

 (Fig. 15) , with Treponema (Leptospira) ictero-hemorrhagiae, 

 the cause of infectious jaundice, and Treponema (Leptospira) 

 interrogans, said by Noguchi to be the cause of yellow fever. 



The life histories of the treponemas are still matters of 

 controversy. In these minute forms some observers maintain, 

 others deny the existence of reproductive ("coccoid") bodies 

 such as are found in other spirochaetes. The fact that in some 

 cases the virus cannot be filtered out lends support to the view 

 that some such minute germs capable of transmitting the dis- 

 eases must be formed. 



