Once you've ruled out ME, it doesn't mean the person isn't sick.
But you haven't ruled it out by using an exclusionary diagnosis. You have only found that you cannot diagnose it. Some patients will also have ME. I think that when we have the PEM blood test a lot of these questions will become moot.
Right now we know of one patient with HIV who fails the 2 day CPET with a bad reaction that is typical of ME. I suspect they have HIV and ME, its just that we cannot diagnose ME currently with our limited definitions.
As for exclusionary conditions I think many, including doctors, do not understand what that means. You can have an exclusionary condition and have a diagnosis of ME if:
1. Its being treated but the broad symptoms do not go away, its not just about fatigue, or
2. The symptomology of the other condition does not match the ME symptoms, and ME is still the only viable diagnosis that matches those symptoms.
I would also like to add:
1. The patient fails a 2 day CPET in a manner consistent with ME, or
2. The patient has a clear history of PEM. The only other condition I have seen claimed to have PEM and I am not sure there is a confusion between exercise intolerance and PEM, is primary biliary cirrhosis, and I am not sure about that either.
Its probably possible to have ME and any other disease. The prevalence of such comorbidities may not be the usual though.
For example, from what I have read, and recently I read something from someone else who found the same data, its rare or very rare for anyone with ME to get atherosclerosis. Heart failure, valve failure, myocarditis, yes; atherosclerosis, no. In fact I only knew of two who did, and both were smokers.
Its an issue of failure to diagnose because of exclusion, which means that many patients will get bad advice or no help at all. Its just not easy to work around this, though I suspect that will change fast in the next few years given how many new investigatory tests are under development for ME.