600 TREPONEMATA, SP I RON EM AT A, LEPTOSPIRATA 
Medical Corps of the United States Army, carried out a series of 
experiments never excelled from a scientific standpoint, which showed 
conclusively : 
1. The virus of yellow fever circulates in the blood stream of a 
patient at least three days after the initial chill. An injection of blood 
from a patient at this stage of the disease will reproduce the disease 
in a non-immune. 
2. The virus will pass through a Berkefeld filter; it belongs, therefore, 
to the group of filterable viruses. Berkefeld filtrates of the blood 
will establish the disease through a series of cases, thus indicating that 
a living virus is being perpetuated. 
3. The disease is transmitted ordinarily by the bite of a female 
mosquito belonging to the genus Aedes. The insect is now known 
as Aedes segypti.^ 
4. A patient is infective for a mosquito only during the first seventy- 
two hours after the initial chill and onset of the disease. 
5. A latent period, during which the insect is non-infectious, must 
elapse before the disease may be transmitted to a non-immune subject 
through the bite of the yellow fever mosquito. 
6. One attack appears to confer lasting immunity, provided the 
individual resides continuously in the tropics. 
The two cardinal features of the transmission of yellow fever— 
infectivity of the patient during the first three days of the disease, 
and the part played in its transmission by the mosquito, Aedes aegypti 
(calopus), were immediately put to the acid test of practical sanitation 
by Gorgas,^ first in Havana and later in Panama, where he organized 
and directed the sanitation of these pestilential cities along lines which 
soon freed them from yellow fever and other diseases of endemic 
origin as well. 
The etiology of yellow fever has at various times been ascribed to 
a micrococcus (Cryptococcus xanthogenicus, Freire^), a bacillus 
(Bacillus icteroides, Sanarelli''), a filterable virtls,* a protoxoan (Para- 
plasma flavigenum, Seidelin^), spirochsetes (Spirochseta interrogans, 
Stimson''), (Leptospira icteroides, Noguchi^). Of these, only two 
appear to be of significance, the spirochetes of Stimson and of Noguchi. 
Stimson observed his organism in sections of the kidney of a yellow 
fever cadaver, stained by the method of Levaditi. Inasmuch as it 
possesses striking resemblance to the organism isolated and studied 
• The original name of the insect was Culex fasciatus; it has been changed successively 
to Stegomjaa fasciata, Stegomyia calopus, to Aedes calopus, and finally to Aedes 
segypti. 
2 See Jour. Am. Med. Assn., 1906, 46, 322, for brief summary. 
3 Doctrine Microbienne de la fi^vre jaune, Rio de Janeiro, 1865. 
* Ann. Inst. Pasteur, 1897, 11, 433. 
•> Philadelphia Med. Jour., October 27, 1900. (Complete work of the Yellow Fever 
Commission will be found in Senate Document No. 822, January 27, 1911). 
« Yellow Fever Bull., Liverpool, 1911, 1, No. 7. 
^ PubHc Health Report, 1907, 22, 541. 
8 Jour. Exp. Med., 1919, 29, 585. 
