The clinical history of the disease indicated a general blood infection.  As my equipment included the best microscopical apparatus made, I had strong hopes that in properly stained preparations of blood taken from the circulation of yellow fever patients my Zeiss 1-18 oil immersion objective would reveal to me the germ I was in search of.  But I was doomed to disappointment.  Repeated examinations of blood from patients in every stage of the disease failed to demonstrate the presence of microorganisms of any kind.  My subsequent investigations in Havana, Vera Cruz, and Rio de Janeiro, made in 1887, 1888, and 1889, were equally unsuccessful.  And numerous competent microscopists of various nations have since searched in vain for this elusive germ.  Another method of attacking this problem consists in introducing blood from yellow fever patients or recent cadavers into various “culture media” for the purpose of cultivating any germ that might be present.  Extended researches of this kind also gave a negative result, which in my final report I stated as follows: 

    The specific cause of yellow fever has not yet been demonstrated.

It is demonstrated that microorganisms, capable of development in the culture media usually employed by the bacteriologists, are only found in the blood and tissues of yellow fever cadavers in exceptional cases, when cultures are made very soon after death.

Since this report was made, various investigators have attacked the question of yellow fever etiology, and one of them has made very positive claims to the discovery of the specific germ.  I refer to the Italian bacteriologist, Sanarelli.  His researches were made in Brazil, and, singularly enough, he found in the blood of the first case examined by him a bacillus.  It was present in large numbers, but this case proved to be unique, for neither Sanarelli nor any one else has since; found it in such abundance.  It has been found in small numbers in the blood and tissues of yellow fever cadavers in a certain number of the cases examined.  But carefully conducted researches by competent bacteriologists have failed to demonstrate its presence in a considerable proportion of the cases, and the recent researches of Reed, Carroll, and Agramonte, to which I shall shortly refer, demonstrate conclusively that the bacillus of Sanarelli has nothing to do with the etiology of yellow fever.

So far as I am aware, Dr. Carlos Finlay, of Havana, Cuba, was the first to suggest the transmission of yellow fever by mosquitoes.  In a communication made to the Academy of Sciences of Havana, in October, 1881, he gave an account of his first attempts to demonstrate the truth of his theory.  In a paper contributed to The Edinburgh Medical Journal in 1894, Dr. Finlay gives a summary of his experimental inoculations up to that date as follows: