238 THE FILTERABLE VIRUSES 



only when there are present stegomyia capable of conveying the 

 parasite of the disease. 9. The spread of yellow fever may be 

 prevented by destroying the stegomyia and preventing egress and 

 ingress of the insects from yellow fever patients to the non-immune. 

 10. No insect, other than the stegomyia, has been found to be con- 

 cerned in the spread of yellow fever. 



Yellow fever is a tropical or subtropical disease, because the 

 stegomyia 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 stego- 

 myia are then either killed or undergo hibernation. Many conclu- 

 sive experiments by Reed and Carrol, by Guiteras, and by the 

 French Commission have proved that the stegomyia is beyond 

 doubt the cause of the spread of the disease. No immunity, other 

 than the actively 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 

 micro-organism. 



Certain protozoonoid bodies have been seen by numerous observ- 

 ers, notably by VanderLoeff, L. Peiffer, and Guarnieri. The latter 

 gave the name Cytoryctes vacciniae s. variolae. In the deep 

 layers of the epithelial cells of the pustules of vaccinia and small-pox, 

 in the experimental lesions on the corneae of rabbits, and in the proto- 

 plasm of the cells, these bodies are found. They are about the size 

 of a micrococcus and exhibit amoeboid movements in hanging drop 

 preparations. They are perfectly characteristic of the lesion pro- 

 duced in vaccinia and are not found in other diseased conditions. 

 Although championed by the great authority Prowaczek, their 

 protozoal nature is not accepted by all authorities. 



In variola many different changes occur in the appearances of 

 these cytoryctes, suggesting developmental cycles. 



In variola they are often intra-nuclear, while in vaccinia they are 

 never found within the nuclei. 



