Well the flu we had at christmas went like this:
dd (age 5) - terribly ill. Temps over 40, shivers and aches. No appetite at all. Stopped peeing for a worrying time while I desperately tried to get fluids into her (but then started again). Had to be carried to loo when applicable - mostly just lay in bed, in dark, and slept for about 20 hours out of 24.
ds (age 3) - not so bad. Temps but not as high nor for so long. Very lethargic. No appetite. Slept and slept.
DH - felt like a bad cold. Temperature. Sore throat.
Me - high temps for about 4 days. Exhausted. Headache. Shivers and aches. No appetite.
So on the basis of that, would feel that everyone reacts differently to what can essentially be the same virus, and what knocks one person completely for 6, such that they couldn't get out of bed if there was a million pounds on the front lawn, might not feel so bad to someone else.
I got rather cross with dh at the time because he said it couldn't be flu because he wasn't feeling like death warmed up.
I know that expat had it, and that the severity of the symptoms varied a great deal within her family, and they were confirmed (swabbed) cases.