Discovery of the “Negri bodies” in rabies, the size of which makes viewing possible with a light microscope, and allows prompt microscopic diagnosis. German translation in Z. Hyg. InfektKr., 1903, 43, 507-28. 
Even though Remlinger proved in 1903 that rabies is a virus, until 1909 Negri tried to prove that the intraneuronal inclusions named after him corresponded to steps in the developmental cycle of a protozoan.