Any person who doesn't live on a desert island has a virus present in them in some form. For example once you catch a herpesvirus, it never goes away from the body. More than 90% of the population carry HHV-6, a great majority have been infected with EBV, HSV1/2 are common, CMV, enteroviruses, etc. as well. The question is are these viruses doing any harm, in other words, are they active in some manner in ME/CFS patients? Here we don't have a clear answer I'm afraid.