I would not be surprised at all to find many cancers and other illnesses, including neurological and even presently classified psychological illnesses, end up being caused by retroviruses.
Medical knowledge of retroviruses is fairly recent. Although retroviruses in animals have been known since 1908, the first human pathogenic retrovirus, HTLV was discovered in 1981. HIV was discovered in 1983. Think of that. The AIDs epidemic was early 1980s. The CFS outbreaks reported to CDC were in mid 1980s.
In the 1970s, only 40 years ago, within my lifetime, Nixon's "war on cancer" failed to find any retrovisuses were causing cancer. Of course, that is incorrect.
So while we speak of being in early stage of research into XMRV, we're only in the toddler stage of understanding human retroviruses in general.
XMRV has some unique properties. It breaks some paradigms. Now, how many other retroviruses are in humans and causing disease but we haven't found the right code to unlock its secrets. It is like these viruses write in invisible ink and researchers have to find the right chemicals to make that invisible ink show up.
Except for CFS, which had clear indications of possible retrovirus cause, the scientist don't have any reason to believe any ink is there. Imagine you get a piece of paper with block dots on it. No understanding. You try to figure out what those dots mean. Why would you think invisible ink was used on the piece of paper? So why would you even work to find the chemical recipe to reveal it. You might be trying to connect the dots, inverting them, turning the paper upside down, doing everything else but thinking invisible ink is on the paper.
So, I will not be surprised if retroviruses are found at the root of many illnesses, since we know very little about human retroviruses and have had to change previously held beliefs about them as recent as eight months ago.
Tina