The answer to that question nobody knows, but it has been discussed on this forum from time to time.
My hunch is that it may be the robustness of immune response at the
time of contracting an acute enterovirus infection that plays a major role in determining whether or not that infection becomes chronic and leads to ME/CFS.
I got this idea from
Dr Chia's observation that people inadvertently given corticosteroids (which tone down the immune response) during the time of an acute viral infection have a higher risk of developing ME/CFS. Chia says he sees lots of ME/CFS patients who have this medical history of corticosteroids given during acute infection.
It seems that acute infection + corticosteroids is almost a recipe for creating ME/CFS.
My feeling is that if the immune response is weak and inadequate during the fierce initial acute viral infection, then the virus may penetrate deeper into the body, breaching into tissue compartments such as the central and peripheral nervous system, and then once the virus has insinuated itself into these compartments, it both triggers ME/CFS and also becomes harder to eradicate. Three brain autopsies performed on ME/CFS patients found enterovirus infection in the brain tissues.
I have also speculated that if the immune response is weak and inadequate, it may allow the virus to enter and infect important immune organs such as the thymus (coxsackievirus B4 can infect the thymus), or important immune cells such as B-cells (in mice, acute coxsackievirus B has been shown to infect up to 10% of B-cells). Then once this happens, it may weaken the immune system or render it dysfunctional, such that it now has difficulty clearing enterovirus from the body.
As we were
discussing just a few days ago, it is interesting that in males, oxymatrine can lead to permanent improvements in ME/CFS (improvements that hold even after stopping oxymatrine), which suggests some hysteresis is going on: ie, that with a severe enterovirus infection, the infection itself might weaken the immune system and thus prevent viral clearance; but once that infection is reduced with oxymatrine, the immune system weakening is abated, and so thereafter, the immune system can better keep the virus in check.
As an aside: I wonder whether it might be possible to create a
mouse model of ME/CFS by exposing mice to enterovirus while they are given corticosteroids. If we could create ME/CFS in mice, that might be tremendously useful for research purposes.