According to leading researchers like Dr John Chia and Dr Jonathan Kerr, these two infectious agents are considered known causes of ME/CFS:No. They are considered to be triggers of ME.
Chronic Chlamydia pneumoniae infection: a treatable cause of chronic fatigue syndrome.
Chia JK, Chia LY
Chronic fatigue syndrome and arthralgia following parvovirus B19 infection
Kerr JR, Bracewell J, Laing I, Mattey DL, Bernstein RM, Bruce IN, Tyrrell DA.
Successful Intravenous Immunoglobulin Therapy in 3 Cases of Parvovirus B19–Associated Chronic Fatigue Syndrome
J. R. Kerr, V. S. Cunniffe, P. Kelleher, R. M. Bernstein, and I. N. Bruce
More proof that parvovirus B19 and Chlamydia pneumoniae cause ME/CFS also comes from the fact that these pathogens are treatable, and once you treat these infections, the ME/CFS symptoms disappear.
Why do some who have the infection, not develop the disease state that is known as ME?
Why do most individuals (90%) who catch poliovirus show no symptoms at all (asymptomatic), but a very small percentage (1%) will get paralytic poliomyelitis from the poliovirus?
Answer: because there are obviously other factors involved, and these other factors determine whether poliovirus causes paralytic poliomyelitis or not.
However, in spite of the fact only a small percentage develop paralytic poliomyelitis from poliovirus, nobody would argue that poliovirus is not the cause of paralytic poliomyelitis.
Likewise, if enterovirus does turn out to be a major cause of ME/CFS, that does not imply that enterovirus will always cause ME/CFS in everyone who catches it. Even HIV does not cause AIDS in everyone, because certain individuals are largely immune to HIV.