The Mac is not immune to viruses, Trojans, backdoors, adware, spyware and other unwanted applications. In reality, the Mac OS, its included applications, and third-party applications have and will continue to have security issues that can allow some form of attack. Anti-virus programs don’t just provide protection against known viruses; they also include anti-phishing, anti-adware, anti-spyware, and other tools that can keep your Mac from picking up debris as you browse the web. It is therefore strongly recommended that you install anti-virus software on your Mac.
The University of Edinburgh provides mac anti-virus software to entitled users to protect their home and non-supported desktop macs from being infected with malware. For more information on Sophos anti-virus software, including how to download and install it, check the Information Services website. There is a version for Mac OS 10.4 and also an older version for Mac OS 10.2 and 10.3 machines.