Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) was originally dismissed by a lot of community doctors as well as more academic medical people. There are still a few who don’t believe in it and dismiss it as a behavioural or attitude problem. Thankfully those people are in the minority now. Unfortunately that doesn’t mean they’re not in influential positions.
One surprising contributor to validating ME/CFS is long covid, which seems to be the same condition but catalysed by a different virus.
I’m not a medical expert and could have mistakes in the above post but it’s generally correct.