Not to be flippant, but most of us don't really have the flu when we think we do. Most of it is just upper respiratory thingeys. The flu normally kicks the life out of you. I had it one year and collapsed when I tried to walk across the room.
I also have severe airborne allergies so I don't play around with this stuff.
I know that. There are a number of strains of flu out there every season too, some worse than others. That's why you can get a flu shot and still get a different strain of the flu that wasn't covered by the shot. Some times it takes weeks or months to get your strength back. Some years our whole family would get it and some of us would be sicker than others.