then the Mac would be able to get one.
Right now there are no such things wandering around outside of a lab.
there are a number of pieces of malware that can, and do, attack OS X but they mostly have to be downloaded and installed by the user.
What the Apple Store probably told you was not that Macs cannot get viruses, but that there are none right now and that they cannot be attacked by a Windows virus.
If you feel more comfortable with AV software installed, take a look at ClamXAV. It's free and uses very few of your machines resources.
Where is that forum that you were looking at?
i just bought a macbook pro and i was told in the apple shop that macs dont need scurity protection as they cant get virus,s,however on a forum i read that they can,can they or not? thanks