James Carroll decided feed an "old" mosquito some of his own blood. Three days later, Carroll was ill with the yellow fever. Jesse Lazear's notebook had noted that the insect had fed twelve days before on a yellow fever patient, who was then in his second day of disease.
Then Lazear was working at the bedside of a yellow fever patient when he was bitten by a stray mosquito. Five days later, he developed yellow fever; on the seventh day of his illness, Jesse died.
The reason we could not interpret our earlier experiments was that we had not allowed for the incubation period of the disease. If we followed this precise course of events, we could transmit yellow fever by mosquitos every time.
We had our proof that mosquitos spread the disease.