The news and blogoshere was wild reporting on the first Trojan horse to affect Mac OS X 10.4 (Tiger) named Leap-A.
This has been categorized as a low-level threat by all of the virus software manufacturers, as it requires the user to download and install it.
Macworld reports on Leap-A (aka Oompa Loompa) providing some sensible advise for users:
The only way you can get the Leap-A malware on your machine is if you take some action to put it there yourself. You might receive a file from a buddy in iChat, or download something from the Internet, or open an attachment to an e-mail message. The program code is presently hiding in what claims to be pictures of OS X 10.5, Apple’s next major OS X upgrade. To get Leap-A on your machine, you must (a) receive the file, which is compressed; (b) expand the archive; and© double-click what appears to be an image file to execute the code. You cannot get the malware by simply browsing the Internet, reading e-mail, or chatting with friends in iChat.
Virus software is available for the Mac, such as Symantec Norton Anti-Virus v.10 Mac
Always practice safe downloading by using web sites that you trust.