If you want to be naive and believe there are no mac viruses in the wild, go ahead, nothing short of your system being compromised will make you believe otherwise, so ingrained is the "belief" that macs are virus free.
Then you know of one, ONE (1), virus affecting Mac OS X and being in public circulation?
It is not a believe I have, it is that other thing, ahem, knowledge. And since I have been running Mac OS X since 2004 and visiting all those sites being called shady (porn, torrents, wares, ...) and clicking away on them, I still not have contracted one virus or any other malware. Probably due to common sense.