In Latin, "virus" is a mass noun, like "stuff", "water", or "poison", its original meaning. There is no attested plural. Being of originally Greek origin doesn't help matters.

In English, there's never a reason to use anything other than "viruses".

> Latin plural is viri

Actually, virus is a "mass noun" (uncountable) and has no plural. If it ever had a nominative plural it would be vira, and out of anything, viri is its genitive singular.

> Word is from Latin - Latin plural is viri.

No, that's the Latin plural of vir; viri means "men" not "poisons" ("virus" means "poison" in Latin.) [1]

http://www.google.com/#q=translate+viri+from+latin+to+englis...

Oops. I mistook the genitive listing in Whitaker for a plural.