The Path Between the Seas

by David McCullough

Yet no yellow-fever cases had ever resulted for Finlay. His notion that the mosquito had to be the transmitting agent was perfectly correct and like Ross in India he had hit upon it alone. What he did not know, what Reed and his staff were to demonstrate, was that Stegomyia fasciata transmits the yellow-fever parasite only according to a very particular time pattern, that the development of the parasite within the insect requires what is known as the period of “extrinsic incubation.” For the mosquito to become infected it must suck the blood of the yellow-fever patient within the first three days after the patient has contracted the disease. Then once the mosquito has taken the blood, another twelve to twenty days must pass before the mosquito can transmit the infection. Finlay had simply been applying his infected mosquitoes too soon.