Mac viruses are there, but you have to go to the back-alleys of the ghetto of the internet. All viruses for mac are a trojan horse. Your local apple store usually has some sort of firewall/antivirus (I can't remember a name)
Here is a quote from one of the forums I use to frequent, but It is being spammed unmercifully. This was from one page that was spammed
Woah, when I loaded this page, a "clear-codec.v.3.5.8.2.dmg" started to download?! WTF a Mac virus?! EDIT: I scanned with VirusTotal, and it is definitely Mac malware, called the DNSchanger/RSplus trojan. WTF. I submitted to ClamAV for inspection...