Too many primarily or exclusively Windows-admins enjoy the schadenfreude of Macs encountering/having/being capable of infection with malware which is a more realistic term than 'viruses.' I'm talking about the current OS not decades ago. Last but not least: 'viruses' for Mac is an interesting item. Sophos free or paid is good, BUT interacts HORRIBLY with Apple Mail, so if that is in use, headaches and aggravation will be unavoidable, and another product is probably a better choice (many Mac users who do use Apple Mail are not quick to give it up). That said, the best (most worthwhile and capable of protection from current real-world threats to Mac OS X) products are the paid version of Malwarebytes, and BitDefender. Same with the *free* BitDefender offering via the Apple AppStore channel. After the demo expires it's a passive (meaning when enacted by the user) check only.

The full-fledged Malwarebytes with active detection/prevention is NOT free. Worthwhile products for the Mac that are free are Sophos and Avast (if you wish to use their product).