I first came across anti virus software in the early 90's and was surprised when I read Norton's list of virus types. There where so many that they listed as not being seen in the wild yet? So how did they know about them? I came to the conclusion that perhaps they where writing code for infection, what better way to create a market than to create a threat.
Ultimately it comes down to the user and how they use the machine they are sitting in front of. I think that in the past, a lot of irresponsible people could not afford or want a Mac, because you can't play games on it (their cry). And a PC is cheaper. (Look mom I'm a computer expert as I've bought and plugged in some components...designed to be plugged in) . These same people are also unwilling to pay for authentic software, so off they go to get cracked copies, which they circulate to all the other freetards, together with its payload, and together with a flawed OS like Windows you have the scene set for mass infection.
I've seen so many people in the corporate environment try to open every type of attachment regardless of where it came from, failing (but not failing to install the virus or Trojan), and then sending it on to other people to see if they can open it?
I firmly believe that the user is the weakest link. Its not helped by an operating system that allows easy installation of any software. In this respect I think that the user base of OS X is more responsible, as if they can afford the kit, they can afford the authentic software. A recent case was of the appearance of either Iworks or Ilife on the torrents, It had a Trojan embedded in it. Since both of the programs only cost £69 each, the freetards got what they deserved. Russian proverb "only free cheese is in mousetrap"
I moved from windows to OS X about 3 years ago and would never go back. Even Norton, Zone Alarm and others failed to protect the machine, when Laurel and Hardy are your castle guards what chance do you have? At least with OS X the castle has a few less open doors.
It always makes me laugh when they have these so called contests to break into a machine. Oh look he cracked it in less than two minutes! (by using the carefully scripted strategy that he worked on for months in advance?). Funny how they always target the Mac? nothing to do with the fact that they get to keep the machine they cracked? After all who'd want a machine running MS? resale value next to nowt.
So as a happy OS X user I use ClamXav occasionally, and for my Parallels & Windows partition (must have my fix of DoD), AVG, as it's just not good form to forward on Windows virus's and Trojans.
People are stupid and too trusting these days. In the past, the Bear, Lion, Wolf, or Local murderer, would have got the trusting idiot, hence keeping the idiot gene reduced in the overall gene pool. These days you see them every where, idiots breeding out of control, covered in tattoos and piercing, baseball caps on back to front (the original IQ minimiser) and they all look like Gregory McCalium