It's been conclusively proven that Macs aren't immune to viruses. If you use a Mac, you really should have security software installed.
Fortunately, you don't need to buy an expensive security program. Sophos Anti-Virus for Mac Home Edition is a powerful, free solution.
It detects and removes viruses written for Macs with little effort from you. Plus, it detects Windows viruses as well, so you don't accidentally pass them on to Windows computers over the Internet, a network or via flash drive.