Who knows? Very little is known about this illness.
She and I came down with same very sudden-onset flu-like illness 6-7 years ago. Maybe it's strictly viral. But then, I had some mild symptoms before that which may have been early ME/CFS.... or something else entirely.
My husband doesn't have ME/CFS despite close contact with us all these years. Why not if it is contagious at all?
My uncle has ME/CFS. He was not around us when we had the flu-like illness, but was already sick then.
Maybe a viral (or retroviral) infection PLUS a genetic factor are necessary for ME/CFS. Maybe it's been passed down vertically in my family somehow.
Other people have completely different experiences and paths to ME/CFS.
We simply don't have sufficient information, yet, to draw any conclusions. We have some interesting theories, but they don't currently answer all the questions about transmission, course of illness, or many other factors.
Bottom line: There is a lot we simply don't know, yet, and as patients we will still have to cope with that ambiguity for some years.
Maybe a viral (or retroviral) infection PLUS a genetic factor are necessary for ME/CFS.
I believe this theory will turn out to be correct through experimentation!
Though I do wonder about "clusters"