Hey I’m a researcher who works on ME (in the past called CFS).

ME/CFS is currently classified as a disease/biological illness according to the CDC.

ME is a disease state in itself. We don’t know much about it, but it can’t be explained by other diagnoses, as the defining factor, neuro-immune abnormalities including immune activation showing up post exertion is unique to it. You’re completely right that we don’t yet have a reliable biomarker. We have a test that differentiates from healthy controls, but it was discontinued for ethical reasons because conducting the test leads to a sometimes permanent worsening of the illness.

In the past it’s been mixed up and jumbled a lot, but the picture is getting clearer.

There have been a few case reports of degenerative forms of the illness. But in general it takes a more classical relapsing remitting pattern. Although even in less bad stages some patients are severely functionally disabled, even bedridden and tubefed. It has a really wide range of severities with the least severely affected able to work part time and walk and travel, while the most severe might not even be able to communicate.