virus is second-declension neuter (genitive singlular viri, not *viroris), therefore the plural should be vira, as recommended by Wiktionary: https://en.wiktion

virus is second-declension neuter (genitive singlular viri, not *viroris), therefore the plural should be vira, as recommended by Wiktionary: https://en.wiktionary.org/wiki/virus#Latin https://en.wiktionary.org/wiki/virus#Latin