Apple has advised its customers to install multiple anti-virus software engines to protect against possible infection.

In a posting on its support site, which has since been taken down, the company recommended three anti-virus programs: Intego VirusBarrier, Symantec Norton Antivirus and McAfee VirusScan. The page can be seen at Archive.org.

It is not known why Apple has taken this step, nor why it has removed the notice, but one possibility is that there may be new viruses in circulation that target the Apple operating system.

"We have seen more activity on the Macintosh malware front this year," said Graham Cluley, senior technology consultant at security firm Sophos.

"For instance, in August Troj/RKOSX-A was discovered, a Mac OS X tool to assist hackers to create backdoor Trojans which can give them access and control over your Mac.

"More recently, and more seriously, we discovered the OSX/Jahlav-A Trojan which has been deliberately planted on websites waiting for Mac users to visit. This is not a proof-of-concept threat. It is real, and regular Mac users can get infected."

Apple's recommendation is surprising considering that the company has made much of the lack of viruses that target its software, much to the annoyance of Microsoft's Bill Gates.

Although security experts agree that the paucity of Apple viruses is down to its small user base, rather than an invincible operating system, many Apple users do not bother to install anti-virus software.