http://www.theregister.co.uk/2006/02/16/mac_os-x_virus/

Antivirus researchers have discovered what's claimed to be the first computer Trojan to infect Apple Mac OS X computers. The malware, dubbed Leap-A, spreads via the iChat instant messaging system as a file called latestpics.tgz that infected machines send to contacts on an infected user's buddy list.

http://vil.nai.com/vil/content/v_138578.htm