@knackers323
1) montoya said in one of his presentations that autoimmunity has gone up in populations that have survived many waves of infectious diseases. We have to exist in a balance between autoimmunity and infection, waves of Infectious diseases are an evolutionary pressure that favours those with aggressive immune systems but leaves you with a population has more issues with autoimmunity.
2) I have no idea how many people suffered from diseases like this in the past. I can see some evidence in the many novels with background characters who are chronically ill, though they are often treated as malingerers by the authors. I know someone else around here once pointed out that old cookbooks (19th and early 20th Century) used to have a section for invalids.
The real question is what sort of evidence would we expect to see if say 1 or 2% of the population had ME or FM or MS or something of the sort, if the state was small and not interested in providing healthcare, monitoring public health, or providing for the poor.
There may be something in church records, I know dimly that there are people who have studied the way that poor relief (if that is the right term) was managed in small communities in the 18th century and earlier (when it fell under the moral economy rather than state policy) and that in the examples I heard about the local churches where centres for organising it.
The thing is that you would have to look at the sort of evidence that comes down to us, determine whether or not it would record the existence of these sorts of illness, and then look for the evidence systematically before forming an opinion about whether these illnesses are new or not.
Otherwise we fall into something like the "medically unexplained symptoms trap", where people who are not omniscient, and ought reasonably know that, and who have not exhausted all avenues of research open to them (not even tried) opine that their inability to explain "x" is proof that it does not exist.