OSX/Leap-A is both a Trojan and a worm, but not a virus.
It first acts as a Trojan in two different ways. One, you download and install software which contains it. For the second, once running, it then acts as a worm and tries to locate iChat members in your contacts list and pops up the admin dialogue on their computer to allow the installation of software. Anyone who is thinking at all will say no to a request to install software when they know they didn't initiate any such action.
About a year ago, Macworld magazine was reviewing what malware there was for the Mac. Of those reported instances, OSX/Leap-A had infected a whole 50 computers. And this after it had been out in the wild for quite some time. Hardly a major effect.
As far as the "hundreds" of viruses on Macs before OS X, that is a wildly exaggerated number. A few dozen is more like it. And every one of them is completely dead as of Leopard, 10.5.x, as they require the OS 9 and earlier Mac OS's to run.
As far as MacDefender and it's variants, Thomas Reed noted a while back that the person responsible for creating it was caught and is now sitting in a Russian prison. That doesn't mean thought that copies of it still aren't sitting out there.
Lastly, you'd be out of your mind to download anything from torrents. There's no such thing as a "safe" torrent, including Pirate Bay. Such sites are how the most damage is done to Macs. Think the Adobe Master Collection for free looks like a great idea? Well, besides the fact you'd be a 100% thief for doing such a thing, but these torrent downloads are stuffed with other payloads you don't know about. Keyloggers, backdoors, etc. When you launch the installer and enter your admin name and password, you give the installer full permission to install anything in that package, including stuff that isn't part of the software you think you downloaded. It doesn't have to ask you again for the malware that is part of the installer package.