A little googling reveals that there were defective nouns and they lacked the plural.

They were of three types:

1. Most proper names: as, Caesar, Caesar; Gallia, Gaul. 

2. Names of things not counted, but reckoned in mass: 
       as, aurum, gold áér, air; tríticum, wheat. 

3. Abstract nouns: as, ambitió, ambition; fortitúdó, courage; calor, heat

Virus looks pretty type 2 to me.