By 1902 Reed knew that the infectious agent of yellow fever was smaller than bacteria, though he did not specifically call it a virus. "In 1898, the passage of an animal pathogen through a Chamberland filter was reported; it is now named foot‐and‐mouth disease virus. In Cuba, serum from a yellow‐fever case was diluted and passed through a Berkefeld filter (of diatomaceous earth and impervious to bacteria). When inoculated into a non‐immune individual it promptly induced an attack of yellow fever. The word ‘virus’ was not used for this case, but because it might be designated as ultra‐microscopic, the infectious agent of yellow fever was compared with that of foot‐and‐mouth disease of cattle" (Clements & Harbach, "History of the discovery of the mode transmission of yellow fever virus," J. Vector Ecol. , 42 (2017) 208-222).

The yellow fever virus discovered by Reed was the first virus discovered that caused human disease.