256 MOSQUITOES OF NORTH AMERICA 
that the filtered lymph held in solution an extremely active toxin, or that the 
causative organism of the disease was so small as to pass through the pores of a 
filter through which even the smallest known bacteria could not pass. The 
latter explanation was accepted. Reed and Carroll decided to make similar ex- 
periments with yellow fever, and Carroll, returning to Havana in August, 1901, 
succeeded in carrying out some very interesting experiments; in the course of 
these it was shown that the filtered blood serum from a yellow-fever patient 
carried the disease to non-immunes. The serum used for the inoculations had 
been slowly filtered through a new Berkefeld laboratory filter. As soon as 
possible thereafter the filter was sterilized by steam and tested as to its effective- 
ness in preventing the passage of bacteria, with the result that it was found to 
intercept Staphylococcus. The opinion of Reed and Carroll, after these experi- 
ments, coincided with those of Loeffler and Frosch in the case of the foot-and- 
mouth disease, and they were inclined to believe that the causative organism of 
yellow fever is of extremely minute size (ultra-microscopic). 
In 1902 a working party of the Yellow Fever Institute of the U. S. Public 
Health and Marine-Hospital Service worked for some time at Vera Cruz, 
Mexico, in the effort to find the causative organism of yellow fever. In their 
report, published in March, 1908, they describe a protozoan parasite by the name 
of Myxococcidium stegomyie which they stated was found with regularity in the 
appropriate mosquitoes that had bitten patients affected with yellow fever. 
These mosquitoes, after different periods of infection, had been killed, sectioned 
and stained. It was the belief of certain members of the party that in this organ- 
ism they had found the cause of the disease. This conclusion was opposed by 
Carroll in a paper read at the Thirty-first Annual Meeting of the American 
Public Health Association, held at Washington, October 26-30, 1903, after ex- 
amination of much material. His conclusions were as follows: 
“1. The fusiform stage of the so-called Myzxococcidium stegomyie of Parker, 
Beyer and Pothier (1903), is not connected in any way with the transmission 
of yellow fever. 
“2. This organism appears to be not a protozoan parasite, but a yeast fungus. 
In its fusiform stage, the only form in which it was constantly present, it shows 
the characteristic budding, staining affinities and vacuolation or spore formation 
of a blastomycete, and it is found with considerable regularity in both male and 
female mosquitoes that have purposely been fed on overripe banana to which a 
pure culture of a wild yeast had been added in the laboratory. 
“3. The organism has not hitherto been found in repeated examinations of 
mosquitoes of the genus Stegomyia that have bitten yellow fever patients in the 
early stages of the disease, when such insects had been fed only on blood, dry 
sugar and water. This statement applies also to mosquitoes that are known to 
have reproduced the disease in human beings.” 
A second working party, sent out by the Public Health and Marine-Hospital 
Service, went to Vera Cruz in April, 1903, and published its report in May, 1904. 
In the meantime the French commission had endorsed Carroll’s conclusion. 
The conclusions, bearing upon the subject, reached by the second expedition to 
Vera Cruz, are as follows: 
“ The cause of yellow fever is not known. 
