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No operating system is completely immune from Malware (Viruses, Trojans, Worms, etc.). With that said, Microsoft has been more suseptible being that it's a larger target.
There has been a Phishing scam lately that has affected Macs that has been identified as Mac Shield, Mac Defender... The best way to avoid any incident is to keep your Software Updates current and avoid websites that are know to harbor Malware.
There's really no "best" virus software as you are only as good as your best defense. Again, you need to keep your virus definitions current or you leave you computer vulnerable.
I've been using Macs for 6 + year and never had an incident. But, I'm pretty careful and practice the above.
Also, be sure to change your passwords, all of them, often.
If you want a AV program for Mac, I've been using Clam
Here's a link: http://www.clamxav.com/