> But several of those turned out to have underlying diseases that had been overlooked, including cancer, pulmonary embolism, and a muscle ailment called myositis that causes weakness and fatigue. The surprise diagnoses suggest that people told they have ME/CFS often receive inadequate care—something many patients already know. “It’s a huge indictment of our medical system that they had to join a research study to find out what was wrong with them,” says Mike VanElzakker, a neuroscientist at Massachusetts General Hospital.

You see this a lot with "diagnoses of exclusion," sadly. In the absence of specific tests, you can only reach certain diagnostic conclusions by ruling out other stuff first, but (particularly in American healthcare) we often don't perform the rigorous well-it's-definitely-not-<x> workup first.

Or, depending on the circumstances, people don't understand that certain conditions are diagnoses of exclusion to begin with and jump to conclusions, like assuming they have IBS when they actually have Crohn's. (Ask me how I know, lol.)

There's the recent rise of "Chronic Lyme Disease," which many physicians have noted is a dubious diagnosis at best, but it seems clear that most of those people are ill—just with something else. And that "something else" could be a positively-identifiable condition that they haven't been tested for yet, or it could be a legitimate diagnosis of exclusion, like ME/CFS.

The problem, in either case, is actually testing people for all those other things first.