Koobface propagates by sending notes to Facebook friends of someone whose PC has been infected. The messages, with subject headers like, "You look just awesome in this new movie," direct recipients to a Web site where they are asked to download what it claims is an update of Adobe Systems Inc's Flash player.

If they download the software, users end up with an infected computer, which then takes users to contaminated sites when they try to use search engines from Google, Yahoo, MSN and Live.com, according McAfee.

The Koobface virus also attacks users of the MySpace social networking site. Variants of the worm include Net-Worm.Win32.Koobface.a. and Net-Worm.Win32.Koobface.b.

See also "Facebook Hit With 'Koobface' Virus" on SmallBusinessComputing.com.

Facebook Hit With Koobface Virus
Facebook's 120 million users are being targeted by a virus dubbed "Koobface" that uses the social network's messaging system to infect PCs.

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Webopedia's "Did You Know?" Article
The Difference Between a Virus, Worm and Trojan Horse.