Vi-rus? We Linux users do not understand your strange terminology
But yeah, it's really not much of an issue. There are ways for virii, worms, and whatnot to do damage, but it's not terribly likely; the kinds of problems that afflict Windows users often have a lot to do with Outlook, Word, or whatever opening up attachments and executing them; most Linux software does not do silly things like that, and even if it did, there'd be less chance of it doing any harm.
So in short, you don't need anti-virus software.