Elderly people have encountered many infections in their lifetime and have antibodies to them. In flu epidemics it is young healthy people who die, not old healthy people as they have some immunity to the bugs.
ME was recognised in epidemics which were often associated with polio epidemics going back to the 30s. Having had an epidemic of ME seemed to give some protection in polio epidemics.
All the enteroviruses are very closely related, in fact laboratory testing for them just looks at enterovirus species and only differentiates them later. It s an accident of histiry that they have different names whereas all the E.coli species which are not so closely related have the same name but different numbers.
Like many epidemics, it became apparent that it could also happen sporadically. Polio, the disease, is an abnormal reaction to a virus which causes a respiratory infection. This seems to happen more often when a person is older when it is encountered, as happens with many childhood illnesses. It was a scary disease as hit people in better areas rather than the slums where most diseases were found. There was polio A and B and abortive poliomyelitis but the research finished when the vaccine went into general use as it was so effective.
Basically, there were no experts left. Some, such as the Behans and Eleanor Bell carried on looking at ME but the expertise was lost. Virology in general was neglected by the eighties, though HIV and PCR brought it back. Medical students still do not get the training in virology, or indeed microbiology, they got in the 70s.
We now know that enteroviruses are sneaky little beggars that can cause chronic infections because of 2 tricks. They do not burst the cell open when they replicate so they stay out of the way of the immune system and they do not copy true so the immune system does not recognise them in the blood.
When CFS was invented the previous research in the UK was ignored. PEM was not considered for decades, though that has been rectified but the emphasis on enteroviral infection was just dropped and has not recovered whether because it is not "sexy" enough or because the expertise had been lost in the US.
Whether it is only a few of us affected or a major cause it is sad and frustrating that we are hardly any further forward than we were in 1980, not because research has shown it is a dead end but because of neglect.