I don't think this one is widespread, but based on the stealth-like nature of rootkits, it's probably both difficult to detect and remove.

http://news.zdnet.com/2100-1009_22-5920403.html

A worm found spreading via America Online's Instant Messenger is carrying a nastier punch than usual, a security company has warned.  The unnamed worm delivers a cocktail of unwanted software, including a so-called rootkit, security experts at FaceTime Communications said Friday. A rootkit is a tool designed to go undetected by the security software used to lock down control of a computer after an initial hack.