II. PRELIMINARY THOUGHTS ON THE PROBLEM OF THE 
ETIOLOGY OF YELLOW FEVER 
Before setting out upon such a mission as that of the investigation of the etiology of 
yellow fever, it was necessary to frame a sort of programme of the possibilities and the 
modes to be adopted in dealing with the same. 
At the outset it appeared that the true infective agent of yellow fever has as yet 
escaped recognition. Though yellow fever is by no means a virgin field for investi- 
gation, there has been little or no agreement between different observers, in regard to 
their respective claims. The writings of some authors hardly give evidence that their 
methods were sufficient or accurate enough to be of value. 
The study of yellow fever must be begun again from the commencement, and 
any organism which may be found will not only have to be constantly found, but it 
will also have to afford an explanation why it has escaped the attention of other 
observers. That a yellow fever microbe exists there can be no question. Whether 
it be protozoal or bacterial in nature, it will be necessary to endeavour to define the 
mode of transmission and its mode of life in the outer world. The possibilities which 
may be encountered with either a protozoal or bacterial parasite, suggest a number of 
themes for consideration. 
By prejudice there may be an inclination to feel that either kind ot parasite may 
be involved. The epidemiology of the disease (prel. note) is suggestive of the action 
of an intermediary transmitting agency ; this perhaps might better have been referred 
to as a transmitting agent rather than as an intermediate host. 
In the light of our knowledge of other diseases due to animal parasites, it is 
highly suggestive that this form of parasite would occur ; the apparent limitation of 
the geography ot the disease is likewise suggestive, but though no lasting and endemic 
hold has been taken in northern climes, the inability to do so has possibly been rather- 
overrated by writers of text-books. 
On the other hand, the onset and features ot the disease, its rapid course and 
the subsequent immunity are all more in accordance with our knowledge ot bacterial 
diseases. While granting that we know comparatively little of diseases due to pro- 
tozoal parasites, the hope would rather be for the establishment of a bacillary agent 
which might offer the opportunity for therapeutic treatment (as by an antitoxin) ; on 
the other hand, a priori none such would be likely for a protozoal disease in which 
treatment as yet is based on mere empiricism. 
In hunting tor protozoal parasites, experience with the parasite of Tsetse disease 
and of malarial fever should prove useful. It will be necessary to search everything 
in the fresh state — blood at various stages of the disease, and the organs as soon after 
death as possible. As with the Tsetse parasite the centrifuge may be useful in separating 
the protoplasmic structures in the blood from the heavier red blood corpuscles. 
