Yes...let's take a look at the Apple site shall we...and what do we find?
The reason Apple sells Norton is to quiet all the windows switchers that ask for an Anti-virus program when they are buying the computer. Not some real threat. And hey..they make some money on it too. But clearly it is not needed. If it were, millions of Macs would be infected since most do not have Norton (or similar) installed. If there were real threats, not only would Macs be infected all over the place, but Apple would be pre-installing an Anti-virus program with every Mac sold. Since they don't, and since they even crow about the lack of virus susceptibility right on their website, what you are saying just isn't the case.
If you (mistakenly) think that the you can't get a virus on a mac" adage died years ago, then you need to go visit Apple's site:
Apple - Why You?ll Love a Mac - It has a better operating system.