The problem with virus following normal 2nd declension forms is that it conflicts with vir in every case except the nominative singular. Also, I don't see why you'd reject neuter gender for a 2nd declension noun. Most, if not all, 2nd declension nouns are either masculine or neuter, and, while my Latin text says that all 2nd declension nouns ending in -um are neuter, it does *not say that all 2nd declension nouns ending in -us are masculine. All three dictionaries I have say virus is neuter.