Just to add to the post-viral syndrome chorus, this happened to me when I was 20. I caught what seemed like a pretty normal flu, but it eventually morphed into bronchitis and then pneumonia because I was an idiot who thought he was immortal and continued going out clubbing and staying out late with friends and it was four months before I finally saw a doctor and took the advice to rest and get blasted with high dose antibiotics.

I was down for about two additional months after that, in which I caught another flu twice, and could barely get out of bed for a while. And then I proceeded to have complications from colds and flus that evolved to bronchitis or pneumonia every single flu season for the next 8 years.

It took nearly a decade to really recover from that. I see no reason to think any sufficiently bad respiratory infection with high enough viral load wouldn't do the same thing, especially to people much less healthy than 20 year-old me, a two-time state champion cross country runner in high school.