Same here. I used Win 95 through XP without ever installing AntiVirus or AntiSpyware, and I never had malware.
...that you knew about.
I suppose if it is unnoticed, it is symbiosis -- like those hordes of bacteria (and viruses?) that live within and without all humans. (Assuming any remote harm would be noticed too.)
My antivirus is that I usually boot my MacBook to Ubuntu. For both Ubuntu and OS X I stay up to date with security updates. And hope for the best.