The symptoms of human influenza are clearly…
417 BCE to 406 BCE
Although the virus seems to have caused epidemics throughout human history, historical data on influenza are difficult to interpret, because the symptoms can be similar to those of other respiratory diseases.
An overall lack of data up until 1500 precludes meaningful search for the influenza outbreaks in the more distant past.
Possibly the first influenza pandemic occurred around 6000 BCE in China. (E. Mordini, M. Green, eds. (2013). Internet-Based Intelligence in Public Health Emergencies: Early Detection and Response in Disease Outbreak Crises. IOS Press. p. 67)