Contrary to common knowledge, Apple products are not immune to viruses and malware. As Apple products and OS X become more and more popular, so does the creation of new viruses for the platform. Fortunately, there is a reliable antivirus program for Mac OS X. ClamXav is easy to install and easy to use. The program can be downloaded here. Once downloaded, the site also has a guide for installation and use.
Link to ClamXav guide.