Such was the condition in Philadelphia in 1793 and, as far as methods of control of the disease were concerned, there was practically no advance during the last century. The dominant theory was that yellow fever was spread by fomites, that is, exposed bedding, clothing, baggage, and the like. As late as 1898 a bulletin of the United States Marine Hospital Service stated:
"While yellow fever is a communicable disease, it is not contagious in the ordinary acceptance of the term, but is spread by the infection of places and articles of bedding, clothing, and furniture."
Based upon this theory, houses, baggage, freight, even mail, were disinfected, and the most rigid quarantine regulations were enforced. The hardships to which people of the stricken regions were subjected and the financial losses are incalculable. And withal, the only efficient check upon the disease seemed to be the heavy frosts. It was found that for some reason, the epidemic abated with cold weather,—a measure beyond human control.
It is not strange that among the multitude of theories advanced to explain the cause and method of dissemination of the disease there should be suggestions that yellow fever was transmitted by the mosquito. We have seen that Beauperthuy (1855) clearly urged this theory.
More detailed, and of the greatest influence in the final solution of the problem were the arguments of Dr. Cárlos Finlay, of Havana. In 1881, in a paper presented before the "Real Academia de Ciencias Médicas, Físicas y Naturales de la Habana," he said:
"I feel convinced that any theory which attributes the origin and the propagation of yellow fever to atmospheric influences, to miasmatic or meteorological conditions, to filth, or to the neglect of general hygienic precautions, must be considered as utterly indefensible."
He postulated the existence of a material transportable substance causing yellow fever,—"something tangible which requires to be conveyed from the sick to the healthy before the disease can be propagated" and after discussing the peculiarities of the spread of the disease and the influence of meteorological conditions, he decides that the carriers of the disease must be sought among insects. He continues:
"On the other hand, the fact of yellow fever being characterized both clinically and (according to recent findings) histologically, by lesions of the blood vessels and by alterations of the physical and chemical conditions of the blood, suggested that the insect which should convey the infectious particles from the patient to the healthy should be looked for among those which drive their sting into blood vessels in order to suck human blood. Finally, by reason of other considerations which need not be stated here, I came to think that the mosquito might be the transmitter of yellow fever."