There is an inherent problem with any anti-virus software, it cannot detect an unknown virus. In spite of advertising claims, virus detection depends upon detecting the "signature" of the virus therefore the virus must be known so the AV program can know what its "signature" is. This is dependent on a "signature" file that to be effective must be updated daily if not more often. Since there are no known viruses for OS X the only viruses AV software can detect are Windows viruses and unable to infect OS X. There have been a few trojans that can infect OS X but a trojan depends on the ignorance or cupidity of the user to download and install the trojan. As these trojans are installed just like any application, AV software cannot tell the difference between a desired application and an undesired one. Apple's primary internal virus protection has been directed at immediately releasing updates to OS X to block any identified vulnerabilities.
The ClamX AV mentioned by Virtual1 is, like much of OS X, an open source product. It is generally well thought of because of its low impact on system performance. It does not have any "disinfecting" capabilities, but it can move suspected files to a Quarantine folder. It is available for download from the App Store. However, the last time I heard Apple had withdrawn its recommendation for any anti-virus software because of general ineffectiveness and too often negative effect on system performance. I don't know whether or not that still stands.
