I had flu in January 2018 - in fact according to my Facebook memories, today is the second anniversary of me returning to uni after recovering from it. I don’t ever remember feeling so ill. I’m fairly young, very fit and extremely healthy, it was weeks later though until I felt anything like normal. I should imagine if you had flu, then contracted Covid at the same time, or immediately after, then even for a young heathy person, it would be extremely serious.