Quoted from BBC News

In a note posted on its support site in late November, Apple said it wanted to "encourage" people to use anti-virus to stay safe online. The move is widely seen as a response to the growing trend among cyber criminals of booby-trapping webpages that can catch out Mac users.

Before now Mac users have been largely free of the security problems that plague Microsoft's Windows. Fresh threat The support note recommends that Mac owners install one or more of three anti-virus products. Advice on the site said: "Apple encourages the widespread use of multiple antivirus utilities so that virus programmers have more than one application to circumvent, thus making the whole virus writing process more difficult." Apple recommended users try McAfee VirusScan, Symantec Norton Anti-Virus 11, or Intego VirusBarrier X5.

Link to the original article:

http://news.bbc.co.uk/2/hi/technology/7760344.stm