The history books will tell you that it was only in 1933 that Influenza A virus was shown to BE a virus - yet there appears to me to be very clear evidence that French scientists working in 1918 actually made the first call. From "Quelques notions experimentales sur le virus de la grippe", by MM Charles Nicolle and Charles Lebailly in Annales de l'Institut Pasteur of 1918:
1⁰ The bronchial expectoration of people suffering from influenza, collected during the acute period, is virulent.
2⁰ The monkey (M. cynomolgus) is sensitive to the virus by sub-conjunctival and nasal inoculation.
3⁰ The influenza agent is a filterable organism. The inoculation of the filtrate has indeed reproduced the illness in two of the people injected subcutaneously; on the other hand when given intravenously it appears to be ineffective (two failures out of two tries).
4⁰ It is possible that the flu virus does not occur in the patient’s blood. The blood of a monkey with flu, inoculated subcutaneously, did not infect man; the negative blood result of subject 2 at D, is however, not convincing, the blood route seeming to be ineffective for the flu virus transmission.
(Translated by Mrs Francoise Williamson)
It convinced me. But, as my medical colleagues will be quick to tell you, I'm an amateur in this area. Albeit an enthusiastic one...B-)
Picture courtesy of Russell Kightley Media