Viruses depend on browsing habits as much as anything, so the best answer is probably "we don't know." Browsers like Firefox and Chrome alert you if there is an active infection on a particular site. There is also a nasty ah heck going around to all kinds of sites. We were talking about it a couple weeks ago in Computer Help when it showed up on the Disney site.
I don't run antivirus on my virtual machines, because it seems to really slow those down. BUT, I have a backup image of each, so an infection is no big deal.
I haven't had a virus on a machine in quite a while, but that doesn't stop me from having MSE installed on each host machine, just to be on the safe side.