Here's the name of the most effective and smartest anti-virus protection out there:
It's called Ricky Patel.
You are the frontline security force in charge of making sure that your computer isn't allowed to view/run any potentially hazardous wares, that which OS X hasn't already seen fit to dismiss.
First off, at the moment there are no known active viruses which can affect OS X. A trojan would be more likely (a virus is not a trojan) to attach its self, but the only way it is possible for that to happen, is for YOU to give it permission. What that means is that every .app and .dng file YOU decide to run, needs YOUR permission first.
A lot of people are absolutely careless about their computing habits. And this has been why it is so darned easy to infect a Windows machine. Windows users are usually always running as 'admin', which in effect gives them root permissions, and consequently, exploits a very serious security hole. With a "Nix" OS, the user may run as admin/root, but usually running as such will be for a very temporary amount of time, say.... about the time it takes to input your password and run a new app or installer, and that's it.
Therefore, if you trust the source of whom you're downloading something from, then you shouldn't have a problem. If you don't trust them, then you should demand that they check whatever they are sending with their own anti virus/trojan program FIRST.
While you can actually host a virus which was made for a Windows machine,(while remaining safe from it) it is important to know where you're getting the file from in the first place, and cut off the head at its source.
Doug