Apple OSX is based on BSD which is inherently a secure operating system.  The internal security for operating system is based on a number of factors but the most important is that you do not run as root or as admin depending on what vocabulary you are used to using.  What this prevents is malware or viruses having access to the core of your system.  So any issues would be relegated simply to your specific user profile. Also the fact that previously Apple had a much smaller market share than its PC competitors protected Apple from the majority of now where that is released on a daily basis. This is commonly known as security through obscurity but the end result is the fact that the Apple OSX operating system seemed to be more secure than Windows.

But as Apple has increased its market share it has begun to soften statements on how secure the operating system has been. This is mainly since Apple doesn't want to be held liable when the first worms will show that any operating system is still vulnerable to the smart developer of Malware.  The following are recommended antivirus applications for Apple OSX

Intego VirusBarrier X5: VirusBarrier X5 is the acclaimed antivirus for Mac that is simple, fast and non-intrusive. VirusBarrier X5 protects Macs from viruses by examining all the files that are read or written, as well as watching for suspicious activity that may be the sign of viruses acting on applications or other files. If the program finds any viruses, it can repair the files immediately, display an alert, or put the files into a quarantine zone. Users can run scans either automatically or manually, using VirusBarrier X5's on-demand scanner to check any files, folders or volumes at any time.

Symantec Norton AntiVirus for Mac, which is currently the best selling antivirus application for the Mac. Norton Antivirus automatically detects and removes viruses and worms, prevents virus infected email from spreading, protects against attacks that target software vulnerabilities and delivers industry leading protection in the background, so you can work and play without noticeable impact on performance.

McAfee VirusScan for Mac: McAfee VirusScan for Mac guards against all types of viruses and malicious code, including new and unknown threats, that target OS X. VirusScan for Mac offers unintrusive, effective protection that hunts down and kills viruses, worms, Trojans, and other malicious code as they attempt to infect systems, allowing users to continue to work uninterrupted. Easy to use and always on guard, VirusScan for Mac automatically checks for the latest virus updates, so your protection is always up to date. It is supported by McAfee Avert® Labs, one of the world's top threat research centers.