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'

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".