regard to the transmission of the
recognized filth diseases--typhoid fever and cholera--and yellow fever.
The first-mentioned diseases are largely propagated by means of a
contaminated water supply, whereas there is no evidence that yellow
fever is ever communicated in this way. Typhoid fever and cholera
prevail in all parts of the world and may prevail at any season of the
year, although cholera, as a rule, is a disease of the summer months. On
the other hand, yellow fever has a very restricted area of prevalence
and is essentially a disease of seaboard cities and of warm climates.
Evidently neither of the theories referred to accounts for all of the
observed facts with reference to the endemic prevalence and epidemic
extension of the disease under consideration.

Having for years given much thought to this subject, I became some time
since impressed with the view that probably in yellow fever, as in the
malarial fevers, there is an "intermediate host." I therefore suggested
to Dr. Reed, president of the board appointed upon my recommendation for
the study of this disease in the island of Cuba, that he should give
special attention to the possibility of transmission by some insect,
although the experiments of Finlay seemed to show that this insect was
not a mosquito of the genus _Culex_, such as he had used in his
inoculation experiments. I also urged that efforts should be made to
ascertain definitely whether the disease can be communicated from man to
man by blood inoculations. Evidently if this is the case the blood must
contain the living infectious agent upon which the propagation of the
disease depends, notwithstanding the fact that all attempts to
demonstrate the presence of such a germ in the blood, by means of
microscope and culture methods, have proved unavailing. I had previously
demonstrated by repeated experiments that inoculations of yellow fever
blood into lower animals--dogs, rabbits, guinea pigs--give a negative
result, but this negative result might well be because these animals
were not susceptible to the disease and could not be accepted as showing
that the germ of yellow fever was not present in the blood. A single
inoculation experiment on man had been made in my presence in the city
of Vera Cruz, in 1887, by Dr. Daniel Ruiz, who was in charge of the
civil hospital in that city. But this experiment was inconclusive for
the reason that the patient from whom the blood was obtained was in the
eighth