It is also a symptom of pneumonia.
I had COVID, back before it was fashionable (February, 2020). It took me 10 months to feel normal again (the acute symptoms lasted 2 weeks. I hadn’t been that sick in decades).
Fatigue figured prominently. While I was experiencing acute symptoms, getting up to go to the bathroom basically finished my day.
I thought I had pneumonia, and the really scary stuff stopped, just before I was going to go to hospital. Thank Cthulhu I didn’t go. They would have killed me, and I would have infected a lot of people (as it was, all my family and friends got it. Share, and share alike).