Zero. None. Never.
We JUST had a ton of threads on this. YOU are the best anti-virus. Nothing will happen to your computer that you don't allow. No anti-virus can protect you from actually GETTING the files. They can only clean up afterward.
yup, the problem is you wont know till its too late.... better safe than sorry right? sohos, clamAV and bitdefender offers antivirus for mac, now you decide since its your machine..