I don't know if ME/CFS is relatively new, but it is not difficult to speculate about what would have happened to people with such symptoms throughout history. As horrible as this sounds, if ME/CFS has existed for hundreds or thousands of years, it has probably never been a better time to be alive with it, the further you go back in time the more dismal it gets. Unless of course there is something about modern living that is exacerbating it.
Neurasthenia was a bigger wastebasket diagnosis than CFS. After the decline of neurasthenia diagnoses, there would have been an increase in misguided psychiatric diagnoses instead. You can pretty much guess what would happen those people presenting with ME/CFS symptoms throughout most of the 20th century, after a routine examination didn't reveal an explanation for symptoms, out comes the circular gap-filling arguments about somatisation and depression and anxiety, regardless whether or not you actually exhibited these. This still happens now, but I would rather be alive in 2011 with ME/CFS than 1950. Not to mention the people who couldn't afford to see anyone and had no support whatsoever, see below.
Then think about having such symptoms 500 years ago. Back then the concept of mental illness was a lot cruder, medicine was still in the "dark ages" as well, and possession was still a common accusation, so I'm not sure how they would have interpreted ME/CFS symptoms. How severe the illness was would have obviously dictated the impact on your life. The more severely affected would have relied more on charity, and contrary to all this "there's no CFS in poor countries where people have to work to survive" BS from pseudo-skeptics that is actually contradicted by modern research, many would have simply ended up struggling or dying on the street because they simply couldn't have supported themselves even if their life depended on it, half this forum would probably be in that situation eventually if sent back in time.