There are viruses for mac. There are just not common. I am guessing it is because macs are not that popular and because it is harder/requires more skill to write malicious programs for unix-based operating system.
Wrong. There are no viruses in the wild for Mac OS X at the present. There are two trojans, both of which are easily avoided with a modicum of common sense.
'Security through obscurity' has been proven to be a myth time and time again. If it were true, there would be no viruses for much smaller niche platforms that are even smaller than OS X. And that's not the case. I could go further, but the proof is in the pudding. OS X, and it's foundation, FreeBSD (Darwin) have been around for decades and there has never been a true virus.
There are actually anti-virus for os x. I tried Norton but did not quite like it.
Have one at the moment (cant be bothered to look at the name) but it got a bit too invasive; so I disabled it.
Yes, and they search for Windows viruses to protect other people who should be protecting themselves. And for that bit of generosity on your part, you get the privilege of paying Symantec $50+ on a regular basis and derive ZERO benefit from it. Not to mention that Norton/McAfee/Intego/ESET and the myriad of other AV scareware products are known to be highly problematic and result in severe performance and compatibility issues.
Even if you want to be proactive, chances are that if and when a virus actually does arrive for OS X, these products won't be up to date quick enough to deal with it since the vendors spend all of their time trying to stave off Windows viruses. Chances are you'll know about the virus in time to take appropriate precautions long before these vendors actually get an appropriate definition together to deal with it.
So, long story made short, you don't need an AV product at the moment. If you feel really paranoid, you can download the free ClamXAV and run periodic scans with it.
