Many folks, when discussing the various afflictions that may strike one’s computer at any given moment – afflictions like, say, Windows 98 – will often find the need to refer to a virus in the plural. Sometimes, after all, there’s more than one running around at any given time. (Although when more than one is out there, one is usually a worm or a Trojan horse as opposed to a virus.)
So what word do you reach for when you need to talk about a virus and all its friends? Many writers, either because they had too much Latin in school or because they lived in the desert and became familiar with cacti – more than one cactus – call them viri. Still others get fancy and double the final "i", and flail their readers about the head with virii. Some others eschew the classical endings altogether and use an anglicized plural, viruses.
Since the word virus seems to – and, in fact, does – derive from Latin, it would logically follow that it must have a Latin ending, hence the appeal of the first two options to those who wish to sound pretentious – I mean, educated. But as a wise man once said, logic is simply an organized way of going wrong with confidence. The latter example, virii, would have Ovid spinning in his grave at high speeds, for there is no double-i plural in Latin, unless the underlying root word ends in I as well. If the word virii existed in Latin, it would have to be the plural of the word virius. No such word ever existed.
What, then, about viri? Our modern word virus does come from the same Latin word, which meant slime, poison, or venom. But that word never had a plural, which makes sense, if you try to imagine a group of slimes. In that respect, it is like the modern English word debris, which has no plural. Only in modern times has the word come to mean discrete, countable organic particles, and hence has only needed a plural form in its current incarnation.
In English, how does one pluralize a noun ending in -s? By adding –es, and so we make viruses.
Personally, I’ll keep calling them virii just because it sounds so much more festive.