As a definition as to what 'real flu' is, I follow the guide of a conversation I once had with a doctor. I felt REALLY bad. I'd spent the night hallucinating, being too hot then being too cold, calling out in agony. In the morning my flat mate arranged for the doctor to make a house call. He looked at me, took my temperature (close to 40 if I remember) and said 'Do you feel like you're dying?' My honest answer was 'Yes' he said, 'THen you've got the 'flu.'
If you have had anything even approaching the 'flu I'd have a week off work followed by a week off the bike while you recover to full strength.