You'll find it's not necessary 99% of the time.
I'm on a network full of PC users, and only talk to other PC users (other than the odd Mac user or two).
I would never send an exe to my friends, nor would I ever download one. What's the point?
All other viruses, well, I would see they were viruses before I could send them across to other PCs because the photo of "me" would be a photo; it would say "Preview cannot open this document".
So really, Virus Protection on a Mac is pretty useless.