>the plural is viri in latin...
yes, but that is the plural of vir (=man), whereas virus is of neuter gender, and therefore has the plural vira, see
http://lysy2.archives.nd.edu/cgi-bin/WORDS.EXE?vira
I must admit I never saw this plural in contemporary use. To me, it sounds vastly more elegant than viruses, not to speak of virii, which we should rather not put into circulation.