NationMaster - Encyclopedia: Notable computer viruses and worms

early May: the Sasser worm emerges and causes problems in networks, even interrupting business in some cases.
March 19, Witty was the fastest produced worm to date, with a one day window from advisory to release.
late January: MyDoom emerges, and currently holds the record for the fastest-spreading mass mailer worm. It is designed to attack the websites of Microsoft and the SCO Group with a distributed denial of service attack, while enabling remote control of the infected PC.

2003

late August: Two major worms named the Sobig worm and the Blaster worm begin to attack millions of Microsoft Windows computers, resulting in the largest down-time and clean-up cost ever. The worms have also had political consequences as many companies in several countries start to call for government action to prevent further damages from Windows worms.
January 24: The SQL slammer worm attacking Microsoft SQL Server causes widespread problems on the Internet.

October 26, The klez worm is first identified.
mid-2001: The Code Red worm attacking Microsoft's Internet Information Server is released. It received significant publicity because its payload targeted the White House website (http://www.whitehouse.gov).
mid-2001: The Sircam worm is released, spreading through e-mails and unprotected network shares.

May: The VBS/Loveletter worm, also known as the "I love you" virus appeared. As of 2004, this was the most costly virus to business, causing upwards of 8 billion dollars in damage.

March 26: The Melissa worm is released, targeting Microsoft Word and Outlook-based systems, and creating considerable network traffic.

November 2: The Morris worm infects DEC VAX machines connected to the Internet, and becomes the first worm to spread extensively "in the wild".

(c)Brain, the first virus written for IBM PC compatibles, emerges.
November: The SCA virus, a boot sector virus for Amigas appears, immediately creating a pandemic virus-writer storm. A short time later, SCA releases another, considerably more destructive virus, the Byte Bandit.

A program called Elk Cloner, written for Apple II systems, is credited with being the first computer virus to appear "in the wild"—that is, outside the single computer or lab where it was created.