Viruses are more common on macs these days via many different flash based exploits
It's simply a numbers game with viruses. Hackers had no incentive to create Mac viruses for years due to macs low market percentage. As macs prefentage increased relative to windows hackers have responded by creating viruses at a corresponding rate.
Any more an active firewall and a top notch virus program are mandatory on any machine.
Macs are no where near as safe as they used to be. Maybe a hair safer than windows due to the way their permissions are written.