The Most Destructive Computer Viruses of All Time

Computer viruses can be a nightmare. Some can wipe out the information on a hard drive, tie up traffic on a computer network for hours, turn an innocent machine into a zombie and replicate and send themselves to other computers. If you've never had a machine fall victim to a computer virus, you may wonder what the fuss is about. But the concern is understandable -- according to Consumer Reports, computer viruses helped contribute to $8.5 billion in consumer losses in 2008 [source: MarketWatch]. Computer viruses are just one kind of online threat, but they're arguably the best known of the bunch.

In the good  old days (i.e., the early 1980s), viruses depended on humans to do the hard work of spreading the virus to other computers. A hackerwould save the virus to disks and then distribute the disks to other people. It wasn't until modems became common that virus transmission became a real problem. Today when we think of a computer virus, we usually imagine something that transmits itself via the Internet. It might infect computers through e-mail messages or corrupted Web links. Programs like these can spread much faster than the earliest computer viruses.

We're going to take a look at 10 of the worst computer viruses to cripple a computer system. Let's start with the Melissa virus.

Top 10:Melissa

In the spring of 1999, a man named David L. Smith created a computer virus based on a Microsoft Word macro. He built the virus so that it could spread through e-mail messages. Smith named the virus "Melissa," saying that he named it after an exotic dancer from Florida.

Rather than shaking its moneymaker, the Melissa computer virus tempts recipients into opening a document with an e-mail message like "Here is that document you asked for, don't show it to anybody else." Once activated, the virus replicates itself and sends itself out to the top 50 people in the recipient's e-mail address book.

The virus spread rapidly after Smith unleashed it on the world. The United States federal government became very interested in Smith's work -- according to statements made by FBIofficials to Congress, the Melissa virus "wreaked havoc on government and private sector networks" [source:FBI]. The increase in e-mail traffic forced some companies to discontinue e-mail programs until the virus was contained.

Top 9:ILOVEYOU

The ILOVEYOU virus initially traveled the Internet by e-mail, just like the Melissa virus. The subject of the e-mail said that the message was a love letter from a secret admirer. An attachment in the e-mail was what caused all the trouble. The original worm had the file name of LOVE-LETTER-FOR-YOU.TXT.vbs. The vbs extension pointed to the language the hacker used to create the worm:Visual Basic Scripting [source:McAfee].

Who created the ILOVEYOU virus? Some think it was Onel de Guzman of the Philippines. Filipino authorities investigated de Guzman on charges of theft -- at the time the Philippines had no computer espionage or sabotage laws. Citing a lack of evidence, the Filipino authorities dropped the charges against de Guzman, who would neither confirm nor deny his responsibility for the virus. According to some estimates, the ILOVEYOU virus caused $10 billion in damages [source: Landler].

TOP 8:The Klez Virus

The Klez virus marked a new direction for computer viruses, setting the bar high for those that would follow. It debuted in late 2001, and variations of the virus plagued the Internet for several months. The basic Klez worm infected a victim's computer through an e-mail message, replicated itself and then sent itself to people in the victim's address book. Some variations of the Klez virus carried other harmful programs that could render a victim's computer inoperable. Depending on the version, the Klez virus could act like a normal computer virus, a worm or a Trojan horse. It could even disable virus-scanning software and pose as a virus-removal tool [source: Symantec].

Top 7:Code Red

The Code Red and Code Red II worms popped up in the summer of 2001. Both worms exploited anoperating system vulnerability that was found in machines running Windows 2000 and Windows NT. The vulnerability was a buffer overflow problem, which means when a machine running on these operating systems receives more information than its buffers can handle, it starts to overwrite adjacent memory.

Microsoft released software patches that addressed the security vulnerability in Windows 2000 and Windows NT. Once patched, the original worms could no longer infect a Windows 2000 machine; however, the patch didn't remove viruses from infected computers -- victims had to do that themselves.