. 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.

2. Melissa (1999)

Melissa was an exotic dancer and David L Smith was not only obsessed with her, but 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.

3. 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.

4. 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".

5. 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.

6. 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 to be an organised attack against them.

7. Blaster (2003)

Blaster is a worm 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 Microsoft Corporation. During Symantec's tests an infected machine was observed sending a burst of almost 1 800 emails in a five-minute period.

10.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.

Fin24.com/Symantec