Wow, "virus X"! Sounds so sci-fi.
Ties in with my own personal, soon-to-be viral (in the digital sense) conspiracy theory about ties between CFS/LC corresponding to alien/UFO/UAP sightings and, yes, actual abductions!
(Need to create a TikTok account, lemmee see...)
But do I have your permission to use your term "Virus X"?
(I think I'll have to incorporate the song "Planet Claire" in there somehow, as it just sprang to mind).

But, seriously (for me), apparently "Virus X" was coined (or adopted) more recently in the 80's by the WHO to refer to a hypothetical virus that might appear and cause a world-wide pandemic. It seems to be used that way currently.

What you had then might have been referred to, as far back as the 19th century, as "neurasthenia"! Makes me think of terms such as "a rheumy catarrh" (for a bad cold).
Worth looking at this Wiki page:
https://en.wikipedia.org/wiki/History_of_ME/CFS.
It's a great look back, and reminded me of stuff I had looked up and researched back in the early 90's. It also mentions the apt phrase "Raggedy Doll Syndrome", which is rather delightfully and whimsically descriptive of how many of us sometimes feel.

But, yes, you're correct that this (or some manifestation of this) has indeed been around for a long time, and thank you for that lovely bit of personal history!

p.s. So impressed that you're 85 years young!