Recently a ransomware attack was discovered that affects the Mac OS. The malware would encrypt your hard drive data and only allow access if you paid a fee.
The Ransomware attacked the application Transmission…a BitTorrent Client. You were only vulnerable if you downloaded the full, infected installer from the developer. If you performed in-app updates, you would not experience the issue. Both the developer and Apple have taken steps to block the offending code.
General details can be found here and technical details here.
The built-in Mac OS X Gatekeeper / ProtectX anti-malware software has been updated to prevent future infections from the Transmission app. However, there was a window of time where you wouldn’t have been protected in case you downloaded it then.
There is not a need to purchase and install additional anti-virus software on the Mac. If you are in a cross-platform environment Sophus and other anti-virus software can be helpful to protect files that may attack Windows clients.