sources from a few years back would always be talking about Coxsackie B, an enterovirus
more recent studies found more of a prevalence of regular ol' adenovirus, plus Epstein-Barr. It's possible to take a snip of heart muscle, but that's not guaranteed to give an answer.
AFAIK, antiviral meds won't help.
Or, you might be having an autoimmune reaction. But IIRC immune-suppressing corticosteroids don't help in such a case.
Do you live in an area where there is Lyme? Not a virus, but it can also be a cause of infectious myocarditis.