* The number of viruses - and the amount of damage they cause - continues to grow worldwide. Last year saw 10 percent more viruses than 1998, and the FBI estimated their cost has reached at least $10 billion a year. Below are a few of the more infamous attacks:
1986: First known virus, Brain, believed to be created by brothers in Lahore, Pakistan.
1987: First antivirus software appears.
1988: Morris worm burrows into thousands of computers via ARPAnet, a precursor to the Internet.
1992: Michaelangelo virus scare hits front pages as media predict it could wipe out millions of hard drives. A few thousand are affected.
1998: Hacker group Cult of the Dead Cow releases one of the most successful "Trojan Horse" viruses - packaged as (or hidden inside) programs available for download on the Web.
1999: The Melissa virus causes $80 million in damage to computers worldwide. In December, David Smith pleaded guilty to having created the virus, one of few instances in which authorities were able to catch a virus's author.
2000: Some of the Internet's major Web sites - including Yahoo! and e-Bay - are temporarily brought down by denial-of-service attacks. A Canadian teen is later charged with helping conduct the attacks, although the ringleader is believed to be still at large.
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