1981 Some of the first "wild" viruses - those that are released into the public - are the Apple Viruses 1, 2 and 3. Spread on Apple II floppy disks, the viruses spread through Texas A&M University on pirated computer games.

1983 Fred Cohen defines a computer virus with his dissertation paper Computer Viruses - Theory and Experiments.

January 1986 Two Pakistani brothers, Basit and Amjad Alvi, infect the boot sector of a floppy disk with a virus dubbed "Brain." It is the first to spread on the MS-DOS personal computer system and designed to infect each 360 kilobyte floppy accessed on a drive.

Nov. 3, 1988 Robert Morris crashes 6,000 computers on the Internet with a virus program and is fined $10,000. The federal Computer Emergency Response Team formed in response.

1990 Symantec Corp. releases Norton Antivirus software, one the first and now most widely used anti-virus programs.

1991 "Tequila" is the first major polymorphic virus - one that can change its appearance with each infection, making it harder to detect.

March 6, 1992 The media-induced "Michel- angelo" scare predicts five million computers may be corrupted worldwide. In reality only 5,000 to 10,000 are infected.

Feb. 4, 1996 The first Windows 95 virus is discovered and called "Boza," after a potent and headache inducing Bulgarian liquor. Not particularly contagious, "Boza" can only be spread by exchanging diskettes with programs that have the virus attached.

Aug. 19, 1998 "Strange Brew" - not yet found "in the wild" - is the first virus to infect Java files.

March 26, 1999 The "Melissa" computer virus invades e-mail systems at thousands of companies worldwide. David L. Smith from New Jersey (arrested April 2) was convicted in releasing the virus, which caused $80 million in damages.

June 10, 1999 "Worm.Explore.Zip," a file-destroying computer virus, hits some of the nation's biggest corporations, including Boeing, Ericsson and General Electric.

May 4, 2000 Onel A. de Guzman, a Philippine computer college student, is suspected of accidentally unleashing the "Love Bug" virus, which strikes millions of e-mail systems worldwide. The "ILOVEYOU" virus infects computer networks and hard drives, causing $10 billion in economic damage.

June 19, 2000 The "Stages" virus hits the Internet and propagates via e-mail disguised as a joke about the stages of life. With a ".txt." extension, the virus lures recipients into opening it because up until now text files are considered safe.

Aug. 29, 2000 The world's first virus that targets the increasingly popular wireless hand-held computers is discovered. "Liberty" appears to be a game but can erase programs stored in a Palm PDA. The author of the program says he didn't mean for it to go public and helped antivirus companies detect it.

Feb. 12, 2001 A computer virus pretending to be an electronic photo of teen-age tennis star Anna Kournikova overwhelms e-mail servers throughout Europe and North America. The virus is traced to an Internet account in The Netherlands and two days later police arrest a 20-year-old hacker known as "OnTheFly."

July 23, 2001 U.S military Web sites are temporarily shut down to public access to make sure they are protected against a computer threat known as the "Code Red" worm. The computer virus infected at least 225,000 computers and caused an estimated $2.6 billion in clean-up costs.

Sept. 18, 2001 Known as "Nimda," the word "admin" spelled backwards, a fast-spreading computer worm corrupted corporate computer networks and personal computers first appeared in the United States and spread to Asia and Europe overnight. The outbreak could be more widespread and damaging than the "Code Red" infections.

Dec. 4, 2001 Computer worm "Goner" spreads quickly through corporate and personal e-mail inboxes, deleting system files and clogging networks in the biggest outbreak since the "Love Letter" virus of 2000. People using Microsoft Corp.'s Outlook and Outlook Express, ICQ instant messenger and Internet Relay Chat are affected. Four Israeli teenagers later admit writing and spreading the virus, which hit North America, Europe and Australia.

June 2002 A newly-identified virus is the first ever to infect picture files, making sharing family photos on the Internet a potentially dangerous activity. The virus, dubbed Perrun, is not seen to be infecting computers but worries anti-virus experts anyway because it is the first to cross from program infection into data files, long considered safe from malicious data.

January 2003 A fast-spreading worm cripples tens of thousands of computers worldwide and congests the network for countless others -- even disabling Bank of America cash machines. The attack takes advantage of a vulnerability in some Microsoft Corp. software that had been discovered in July.

June 2003 A destructive virus, known as "BugBear.B," spreads to tens of thousands of consumer computers across the Internet. Its code contains a list of roughly 1,200 Web addresses for many of the world's largest financial institutions, making it the first known Internet attack aimed primarily at a single economic sector. However, industry executives say they are largely unaffected because of tight corporate security.

Aug. 12, 2003 An Internet worm, dubbed "LovSan," spreads rapidly across the Internet, part of a coordinated electronic attack exploiting a serious flaw in Microsoft Corp.'s Windows operating systems. The virus-like infection causes computers to mysteriously restart and launches a coordinated electronic attack against Microsoft Corp.

Aug. 19, 2003 Two worms -Nachi/Welchia and Sobig.F - actively prowl cyberspace. Nachi/Welchia wreaks havoc with Air Canada's airline reservation systems, and masquerades as a patch for the "Blaster" virus. Sobig.F Sobig.F, meanwhile, sets up a secret backdoor that can be used to turn victims' PCs into senders of spam e-mail.

Aug. 29, 2003 U.S. cyber investigators arrest 18-year-old Jeffrey Lee Parson on charges of unleashing a version of the damaging virus-like "Blaster" infection on the Internet. The FBI says that at least 7,000 computers were infected by the Minnesota teenager's software.

Nov. 5, 2003 Microsoft Corp. announces a $5 million reward program to help law enforcement identify and convict those who illegally release worms, viruses and other types of malicious programs. The first two rewards are offered in connection with 2003's MSBlast.A worm and the SoBig virus. Microsoft software is often the target of the most serious Internet attacks.

Jan. 26, 2004 A worm called "Mydoom" or "Novarg" that affects Windows users spreads quickly over the Internet, usually appearing as an e-mail error message. The malicious program sends out infected e-mail messages to addresses found on the computer, at a rate of up to 100 in 30 seconds. It also appears to open up a backdoor so that hackers can take over the computer later.

June 25, 2004 A mysterious widespread attack hits thousands of popular Web sites, according to warnings by government and industry experts. The virus-like infection tries to implant hacker software onto the computers of all Web site visitors. While experts say the attack is significant, it does not appear to cause much interference with Internet traffic.