A/V on Macs to my knowledge has never been a relevant issue except too A/V vendors who as noted make ever wilder claims to entice Mac owners to purchase there products.
Why should this be the case, well making the note that most virus writters operate on a 'My Wang is bigger than Yours' basis there is little street cred in knocking up a virus to attack a Mac O/S and the inherent security of the O/S makes it not really worth the effort so they don,t, they do however make repeated forays in the proof of concept area, mostly at the behest of the A/V vendors, not going to go into that one here, as for Malware ect, again driven by money the small sector of the net using Macs is not worth the investment required to attack it with any certainty of a profit, so again they don,t.
There are a few oddities floating around that attack Mac files but the chances of comming into contact with one of these is remote, so in short, keep your firewall on and upto date and enjoy watching us lowly PC users get smacked around by these fools.