I apparently have sarcoidosis. The last flareup was bad enough to get a diagnosis. But docs think based on my history that I've had it for 20 years.

What I think is viral infections can cause low grade immune system dysregulation. Nominally there isn't anything obviously wrong. But it shows up in feeling crappy and having stress intolerance. It's a problem because medicine doesn't have any tests or treatments for that.

It's totally ass. However some people do get better after a year or two.