As anyone working on a PC will know, viruses are the scourge of the internet. Symantec compiled this list of some of the most notorious threats ever seen online.

1. I Love You (2000): Who wouldn't open an email 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 email 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 26 March 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 of 2003. The threat was so aggressive that it was mistaken by some countries as an organised 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".
6. Code Red (2001): Websites affected by the Code Red worm were defaced by the phrase "Hacked By Chinese!" At its peak, the number of infected hosts reached 359 000.
7. Blaster (2003): Blaster is a worm that triggered a payload that launched a denial of service attack against windowsupdate.com, which included the message, "billy gates why do you make this possible? Stop making money and fix your software!!"
8. Sasser (2004): This nasty worm spread by exploiting a vulnerable network port, meaning that it could spread without user intervention. Sasser wreaked havoc on everything from The British Coast Guard to Delta Airlines, which had to cancel some flights after its computers became infected.
9. Storm (2007): Poor Microsoft, always the popular target. Like Blaster and others before, this worm's payload performed a denial-of-service attack on www.microsoft.com. During Symantec's tests, an infected machine was observed sending a burst of almost 1800 emails in a five-minute period.
10. Morris (1988): An oldie but a goodie: without Morris the current threat "superstars" wouldn't exist. The Morris worm (or internet worm) was created with innocent intentions. Robert Morris claims that he wrote the worm in an effort to gauge the size of the internet. Unfortunately, the worm contained an error that caused it to infect computers multiple times, creating a denial of service.

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