But you also have to ask why almost exclusively the hospital staff fell ill and not the patients? maybe because the patients did less physical work, so the immune system was less vulnerable to the virus? It is well known that often very active people get ME. This can best be explained by the open window phenomenon.
You are referring to the Royal Free Hospital outbreak, where many of the staff developed ME/CFS from the virus, but the hospital patients did not get ME/CFS.
Yes I think that exertion is one possible explanation: that the patients did less physical work than the nurses. It is known that exercise and exertion makes enterovirus infection worse. For example, those with coxsackievirus B myocarditis (enterovirus infection of the heart muscle) fare much worse if they exercise. These patients are always told not to exert themselves until the acute heart infection is over.
I also wonder whether there might have been communal rooms such as canteens or staff accommodation buildings at the Royal Free Hospital, where the staff would spend some time, but patients would not. If those communal rooms were contaminated with for example mold growth (in the 1950s, they did not know about toxic mold), then the staff but not the patients would have been exposed to such a biotoxin.
Another possibility is than another pathogen could have spread through much of the hospital staff in an acute outbreak say for example two years earlier. Because it happened earlier, the staff would carry it, but two years later all the hospital patients would be new, and so would not have this pathogen. That pathogen on its own could have been relatively benign, but it might have set the stage for ME/CFS to manifest when the Royal Free ME/CFS later virus appeared.
And finally, the many different infections can be explained by the fact that they were just triggers (as well as physical stress, mental stress, mold, etc.) that weakened the immune system at the time that the potential retrovirus could break out.
There are are only a handful of infections that are linked to ME/CFS. These are primarily: coxsackievirus B, echovirus, EBV, CMV, HHV-6, parvovirus B19, Chlamydia pneumoniae. And then VZV, Giardia lamblia and Coxiella burnetii are also involved. Possibly influenzavirus might play some role.
But dozens of other infections have never been known to cause ME/CFS. For example, just out of the viruses, the following viruses are NOT linked to ME/CFS: rhinovirus, norovirus, adenovirus, HHV-8, measles virus, mumps virus, rubella virus, rotavirus, bornavirus, parainfluenza virus, metapneumovirus, astrovirus, polyomavirus and numerous others. And there are hundreds of bacteria, fungi and parasites which cause human infections but do not cause ME/CFS.
So the question is, why is ME/CFS only caused by a very limited range of pathogens? What is special about the handful of pathogens that have been linked to ME/CFS? Do they have any common characteristic that might explain why it is only these pathogens can cause ME/CFS (to the best of our knowledge)?