>
> >>> (3) exceptionally in <vir>.
> >> What would cause such an exception?
> > Great frequency of use, most likely.
>
... and we've got vi:rus i, poison, which is surprisingly neuter.
Of course it's a rhotacism of wi:sos and originally a neuter type
genus generis. But why has the word adopted the inflexion of a
masculine thematic?