CAUSE A PROTOZOAN 257 
_ “The Myzxococcidium stegomyie is not an animal parasite. Yeast cells some- 
times simulate the coccidia in form and staining reaction. 
“The infection of yellow fever is in the blood serum early in the disease. 
No abnormal elements that bear a causal relation to the disease can be detected 
in the serum or in the corpuscles with the best lenses at our command. 
“The infective principle of yellow fever may pass the pores of a Pasteur- 
Chamberland B filter. 
“ Particles of carbon visible with Zeiss lenses pass through both the Berke- 
feld and Pasteur-Chamberland B filters. 
“ Because the virus of an infectious disease passes a Berkefeld or Pasteur- 
Chamberland B filter it does not necessarily follow that the parasite which 
passed the filter is ‘ ultramicroscopic, or that it may not have elsewhere another 
phase in its life cycle of large size. 
“The filtration of viruses may succeed or fail, depending upon the character 
of the filter, the diluting fluid, the pressure, time, temperature, motility of the 
particles, and other factors. 
“ The period of incubation of yellow fever caused by the bites of infected mos- 
quitoes is usually three days, sometimes five days, and in one authentic instance 
six days and two hours; but when the disease is transmitted by such artificial 
means as the inoculation of blood or blood serum the period of incubation shows 
less regularity. 
“Yellow fever may be conveyed to a nonimmune by the bite of an infected 
Stegomyia fasciata; but the bites of Stegomyia which have previously (over 
twelve days) bitten cases of yellow fever do not always convey the disease. 
“ Fomites play no part in the transmission of the disease. 
“The tertian and estivo-autumnal malarial parasites will not pass the pores 
of a Berkefeld filter.” 
The causative organism of yellow fever still remains unknown.* 
Carroll, of the American commission, in almost the last paper before his 
lamented death, and which forms the chapter entitled “Yellow fever” in 
volume ii of Osler’s Modern Medicine, summarizes, on page 747, the arguments 
which favor the protozoal character of the organism presumed to cause the 
disease : 
“ Notwithstanding all the efforts that have been put forth in that direction, 
the specific organism of yellow fever still awaits recognition. The arguments 
opposed to the idea that the invisible causative agent of yellow fever is a bac- 
terium are: (1) it has never been recognized nor cultivated; (2) according to 
the French Commission the blood of a patient loses its power to infect within 
two days if exposed to the air, and within five days if air be excluded; (3) the 
absolutely non-contagious and even non-infectious character of the disease under 
conditions otherwise favorable but in the absence of the mosquito. The argu- 
ments in favor of its being a member of the animal kingdom are: (1) its obliga- 
tory parasitic existence in man and the mosquito, alternately ; (2) the necessity 
for the lapse of a definite period, two weeks or more after the ingestion of the 
blood, before the mosquito becomes capable of infecting; (3) the restriction of 
the hosts to a single vertebrate and a single genus of invertebrates which points 
to a definite cycle of development; (4) the rapidity of development of the para- 
site within its invertebrate host is governed by conditions of external tempera- 
* Quite recently, claims of the discovery of the causative organism of yellow fever have been 
published by Dr. Harald Seidelin (Zur Aetiologie des gelben Fiebers. Berl. klin. Wochenschr., 
no. 18, pp. 821-823, 1909 ; Protozoon-like bodies in the blood and organs of yellow-fever patients. 
Journ. Pathol. & Bacteriol., vol. 15, pp. goa 788. 1911; The etiology of yellow fever. Yell. Fever 
Bull., vol. 1, no. 7, pp. 229-258, plate. 1911). 
