Top 10 Computer Viruses and Worms

On the Internet's 40th anniversary, Symantec lists the worst cyber threats.

ByABC News
September 3, 2009, 9:44 AM

Sept. 3, 2009 — -- This fall marks the 40th birthday of the Internet, and to celebrate it, Symantec, the manufacturer of online Security software, has put out a list of the "Top Web Threats in the History of the Internet."

Symantec has an interest, of course, in bringing you this treat. The company hopes the memory of worms gone by will entice you to buy its software to protect yourself.

But it's still an interesting list, and you may enjoy arguing with it, suggesting additions, even remembering times past -- unless, of course, your computer was hit by one of these worms or viruses.

So here goes. The descriptions are Symantec's.

1. I Love You (2000) -- Who wouldn't open an e-mail with "I Love You" in the subject line? Well, that was the problem. By May 2000, 50 million infections of this worm had been reported. The Pentagon, the CIA, and the British Parliament all had to shut down their e-mail systems in order to purge the threat.

2. Conficker (2009) -- The Conficker worm has created a secure, worldwide infrastructure for cybercrime. The worm allows its creators to remotely install software on infected machines. What will that software do? We don't know. Most likely the worm will be used to create a botnet that will be rented out to criminals who want to send SPAM, steal IDs and direct users to online scams and phishing sites.

3. Melissa (1999) -- Melissa was an exotic dancer, and David L. Smith was obsessed with her and also with writing viruses. The virus he named after Melissa and released to the world on March 26, 1999, kicked off a period of high-profile threats that rocked the Internet between 1999 and 2005.

4. Slammer (2003) -- This fast-moving worm managed to temporarily bring much of the Internet to its knees in January 2003. The threat was so aggressive that it was mistaken by some countries to be an organized attack against them.

5. Nimda (2001) -- A mass-mailing worm that uses multiple methods to spread itself, within 22 minutes, Nimda became the Internet's most widespread worm. The name of the virus came from the reversed spelling of "admin."