on under rigorous
precautions had been tried. By these experiments all other causes were
ruled out of consideration.

The discoveries which have been made in the past twenty-five years with
reference to the etiology[73] of infectious diseases constitute the
greatest achievement of scientific medicine and afford a substantial
basis for the application of intelligent measures of prophylaxis.[74] We
know the specific cause ("germ") of typhoid fever, of pulmonary
consumption, of cholera, of diphtheria, of erysipelas, of croupous
pneumonia, of the malarial fevers, and of various other infectious
diseases of man and of the domestic animals, but, up to the present
time, all efforts to discover the germ of yellow fever have been without
success. The present writer, as a member of the Havana Yellow Fever
Commission, in 1879, made the first systematic attempt to solve the
unsettled questions relating to yellow fever etiology by modern methods
of research.

Naturally the first and most important question to engage my attention
was that relating to the specific infectious agent, or "germ," which
there was every reason to believe must be found in the bodies of
infected individuals. Was this germ present in the blood, as in the case
of relapsing fever; or was it to be found in the organs and tissues
which upon post-mortem examination give evidence of pathological
changes, as in typhoid fever, pneumonia, and diphtheria; or was it to be
found in the alimentary canal, as in cholera and dysentery?

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