The Macintosh platform suffers from few virus and malware problems. One of the most recent malware problem affected DNS and was caused by adult websites forcing visitors to install software to view movies. Those of you who did, you earned it.
In fact, many of the malware and virus problems that plague PC users simply don't affect Macs at all. In order to be effective, malware has to be installed deep within the Mac OS architecture which requires administrative authentication in order to be installed.
Beginning with OS 10.6 Snow Leopard, Apple has built in protection that scans decompressed application installers for potential system "tom foolery". It's called XProtect.
Are Macs infallible? No, nothing is when it comes to computers. Until XProtect is fully functional and able to catch everything, the best course of action is to periodically check your system for viruses and malware. A great (and free) program to do this is ClamXAV.
I've used ClamXAV in the past and it's removed Windows viruses from my Mac such as the Word 97 virus. This particular virus lays dormant on Macs but can be propagated back to Windows users.
I've always either had a bootable disc or an external drive to perform these tasks, just to be safe.
Erik