260 THE FILTERABLE VIRUSES 



produced by the bite of a mosquito confers immunity against 

 subsequent infection. (7) The period of infection is usually 

 three days but may be from two to six days. (8) A house or ship 

 may be said to be infected with yellow fever only when there are 

 present mosquitoes capable of conveying the parasite of the 

 disease. (9) The spread of yellow fever may be prevented by 

 destroying the aedes and preventing egress and ingress of the 

 insects from yellow fever patients to the non-immune. (10) No 

 insect, other than the aedes, has been found to be concerned in 

 the spread of yellow fever. 



Noguchi reports the discovery of a tiny spirochaete, 4-9/4 long 

 and .2;u wide, in the blood of yellow fever patients. The organism 

 may be transferred to guinea pigs in which it produces lesions 

 comparable to those of human yellow fever. It may be cultivated 

 in anaerobic serum water tubes and it will pass through a filter. 

 During an attack, either human or artificial in a guinea pig, 

 protective anti-bodies are formed. Spirals are only found in the 

 blood in the first few days of an attack. They do not stain well 

 and can best be seen by aid of the dark field microscope. It is 

 claimed that vaccination with these spirals produces some resist- 

 ance to yellow fever. 



Yellow fever is a tropical or subtropical disease, because the 

 aedes is confined to these regions, and the disease is found 

 in low moist localities rather than those that are drier and higher, 

 from the fact that the mosquito inhabits the former and not the 

 latter. Yellow fever dies out after the first sharp frost, because 

 the, aedes are then either killed or undergo hibernation. Many 

 conclusive experiments by Reed and Carrol, by Guiteras, and 

 by the French Commission have proved that the aedes is beyond 

 doubt the cause of the spread of the disease. No_immunity, other 

 than the activity acquired one, is known. 



Small-pox and Vaccinia. These two diseases must be consid- 

 ered to be but two clinical activities of one unknown specific 

 microorganism. 



