They figured out some of long covid in Japan:

[Historic NY]

6. I had Epstein-Barr virus (EBV) when I was 40

its comes back to haunt me from time to time . Stress, illness seem to trigger it. I recall an older Dr friend at the time reminding me that I had an auto-immune disease, and it will stay with me. When I got it I was out of work for over 2 months with complications.

I've had a couple of sudden out of the blue illness, unable to stand due to severe dizziness, sore throat ,unable to keep anything down including water. Staying in bed was the safest place. In a day or two it would go away. Covid tests negative. seasonal changes seem to be a trigger too.