Back several pages on this topic there was a thread about CFS being an infectious disease. I would consider this more a matter of medical sociology than a hard distinction. It changes over time, even without breakthroughs, as in MS.
Let me point out that cervical cancer was not considered an infectious disease until very recently. Now you can hear ordinary doctors talking about eradicating it via vaccination.
There is already a lecture by Eric Klein titled "Is Prostate Cancer an Infectious Disease?", with XRMV suggested as the agent.
The reason these were not originally perceived as infectious diseases was not simply the lack of an identified infectious agent. (Polio was known to be infectious long before the virus was clearly identified.) What prevented cervical cancer from being classed as an infectious disease was the long lag between infection and appearance of cancer.
I don't have good instincts about many social conventions in areas outside my direct experience. I do have an unusual objective way of illustrating the problem.
My own experience had to do with software reliability (a lost cause in many opinions). There we have complete control of the item being studied. Software does not wear out, or break. If it works to begin with, it will continue to work as long as the hardware supports it. If all this is true, how can you talk about reliability?
The answer is that defects are present from the beginning, but they are not exposed until someone, or some data, exercises an aspect of the program which handles this incorrectly, producing an explicit error. You might think of defects as buried land mines, and exposing them as stepping on them. Predicting the chance of exposing a defect is where reliability comes in. In some cases, the calculated operating time before a defect results in an error in operation is measured in thousands of operating years. When you keep finding such long latency errors, after years of testing, you can be sure the program is riddled with bugs. (One successful product was averaging a steady flow of 4,000 year bugs two years after it went into service. Don't ask me about the estimated bug count for Windows.)
In the analogous case of CFS/ME, we appear to have an initial infection which lasts for life. The latency from infection to symptoms may be short, or it may take years. I suspect the sudden onset cases involve reactivated viral infections. If the virus is Epstein-Barr, you can count on 90% of the population having a latent infection.
Because there are cases with infection and without apparent symptoms of any kind, I'm thinking the initial XMRV infection, by itself, may not have clear immediate symptoms of any kind. This is not unusual; even a devastating disease like poliomyelitis left 90% of infected people without symptoms ever mentioned to a doctor. Virtually everyone in an urban population was exposed to the virus. Many became infected. Some were paralyzed, and some died. The difference in numbers may have ranged between millions infected and tens of thousands hospitalized.
The viral 'strategy' of XMRV which confuses doctors probably serves a different purpose: to confuse the immune system. XMRV looks like a kind of stealth virus that lets other stressors, including viral infections, take the blame for damage it causes. Its close similarity to endogenous retroviral sequences suggests this is a strategy which has worked repeatedly before.
With the assumptions I've stated here, I think it takes no shoehorning to fit ME/CFS into the category of infectious disease. Proof of causation is needed, followed by a change in social conventions for medical nomenclature. I expect this to happen, though I don't know when.
