The flu can be nasty. I had a severe case a few years back, and IMO I never fully recovered from it. I consider myself healthy, but I feel that this flu was like getting five years older within two week.
(H1N1)pdm09 was the worst illness I've had, both in duration and severity. Interestingly, that flu virus mostly affected younger people. I'd be interested in reading a thorough comparison of H1N1 and SARS-CoV-2.