First found on March 26, 1999, Melissa shut down Internet mail systems that got clogged with infected e-mails propagating from the worm. Melissa was not originally designed for harm, but it overflowed servers and caused unplanned problems.
Melissa can spread on word processors Microsoft Word 97 and Word 2000. It can mass-mail itself from e-mail client Microsoft Outlook 97 or Outlook 98. The worm does not work on any other versions of Word, including Word 95, Microsoft Office Word 2003, Word 2004 (Mac), and Microsoft Office Word 2007, nor can it mass-mail itself via any other e-mail client, even Outlook Express or Windows Mail (Outlook Express version in Windows Vista).

This virus traveled via e-mail messages entitled "I Love You," crippling government and business computers in Asia, Europe, and the United States. No immediate cure was known as experts were stunned by the speed and wide reach of the virus.
This virus is very similar to last year's Melissa worm that shut down e-mail systems worldwide. "This worm spreads at an amazing speed," said Mikko Hypponen, manager of Anti-Virus research at F-Secure Corporation in Espoo, Finland. "We got the first report around 9 AM on Thursday from Norway, and by 1 PM we had reports from over 20 countries. We estimate that the total number of infected machines is already in tens of thousands. This epidemic might exceed Melissa in both speed and destructiveness."

The fast-moving Code Red computer worm keeps dodging the high-tech bullets meant to kill it. Among the latest victims: FedEx, AT&T and Microsoft � which for weeks has urged customers to install its patch against the worm. As many as 800,000 computers worldwide have been hit, causing business disruptions and Internet slowdowns. Code Red is the biggest Internet threat since last year's Love Bug.

CIH, also known as Chernobyl or Spacefiller, is a computer virus written by Chen Ing Hau of Taiwan. It is considered to be one of the most harmful widely circulated viruses, overwriting critical information on infected system drives, and more importantly, in some cases corrupting the system BIOS
In September 1998, Yamaha shipped a firmware update to their CD-R400 Drives that were infected with the virus. In October 1998, a demo version of the Activision game SiN was infected by one of its mirror sites.In March 1999, several thousand IBM Aptivas shipped with the CIH virus, just one month before the virus would trigger.

First discovered infecting computers in mid-March 2001, the Magistr virus is considered a high risk threat. Cunningly, the virus sits on the infected user's system for a pre-determined period of time before unleashing its wrath which ranges from corrupting data to erasing critical information found in the system BIOS and overwriting sectors on the hard drive. Compounding the risk, on September 3, 2001 a new variant was discovered that was impervious to signature-based scanners. Reportedly, the new variant is an "improved" version of the original Magistr, making the rendering of its malicious payload far more likely.

Nimda is a computer worm, isolated in September 2001. It is also a file infector. It quickly spread, eclipsing the economic damage caused by past outbreaks such as Code Red. Multiple propagation vectors allowed Nimda to become the Internet�s most widespread virus/worm within 22 minutes. Due to the release date, some media quickly began speculating a link between the virus and Al Qaeda, though this relationship ended up being nothing but a conspiracy theory. Nimda affected both user workstations (clients) running Windows 95, 98, Me, NT, or 2000 and servers running Windows NT and 2000.

The Michelangelo virus is a computer virus first discovered in April 1991 in New Zealand. The virus was designed to infect MS-DOS systems and remain dormant until March 6, the birthday of Renaissance artist Michelangelo.
The virus first came to widespread international attention in January 1992, when it was revealed that a few computer and software manufacturers had accidentally shipped products, for example Intel's LANSpool print server, infected with the virus. Although the infected machines numbered only in the hundreds, the resulting publicity spiraled into "expert" claims of thousands or even millions of computers infected by Michelangelo. However, on March 6, 1992, only 10,000 to 20,000 cases of data loss were reported. The news media lost interest, and the virus was quickly forgotten. Despite the scenario given above, in which an infected computer could evade detection for years, by 1997 no cases were being reported in the wild.