Is rheumatic heart disease that is attributed to systemic streptococcal bacteria sometimes actually an unappreciated viral infection? If so, in what percent of cases?

As is true in all of medicine, there is significant misdiagnosis because of overlapping symptoms and confusing and confounding laboratory findings that may be coincidental in one case just as they might be diagnostic definitively in another. So things are rarely cut and dried. There are, indeed, cases of rheumatic fever that are primarily viral with an overlay of bacterial infection history or presence that can be a kind of red herring. This is highly variable but happens about 15% of the time, that it is a virus that is the real culprit in contributing to morbidity and mortality. This would argue for using an antiviral as a precaution, given the availability of safe supplements, whenever rheumatic fever is encountered.