I think most cases of Atrial Fibrillation are due to heart problems, like high blood pressure and a resulting enlargement of the atrium ... from valve problem, and yes from some diseases (rheumatic fever, syphilis, other..), but I have never hear of an association of virus stimulation... it may simply be my lack of understanding, vis-a-vis "virus/disease"... or my lack of knowledge (with is vast).
You didn't mention age, that is another stimulation, wear-and-tear??