Many of the recent viruses that have damaged global computer networks were created by so called "scriptkiddies" - typically, young males who attack computer systems in much the same way as vandals deface walls with graffiti.
They often leave clues to their identity and boast about their efforts on the Web to gain kudos from their peers.
For example, last week police in Minnesota charged a high-school student, Jefferey Lee Parsons (18), with distributing a new strain of the Blaster internet worm, which exploits a weakness in the Windows operating system and then uses infected computers to set up attacks on Microsoft's technical support website.
Parsons altered the original Blaster worm to create a variant that would attack computer systems.
But some hackers or crackers are professionals who undertake their action for commercial gain. Some try to extort cash from companies by threatening to publicise their slack security.
Earlier this year, Oleg Zezev was sentenced to four years in prison for trying to extort cash from Michael Bloomberg. Zezev broke into Bloomberg's computer system and then e-mailed a threat to the businessman, and current mayor of New York, to publicise the break in order to damage Mr Bloomberg's reputation.
Security experts believe the author of the Sobig virus had considerable knowledge of how the internet works. Many within the industry have linked this particular virus to "spammers" - people who distribute tens of millions of unsolicited adverts by e-mail every day.
Some high-profile viruses:
On November 2nd, 1988, the first virus spread through the internet was released by Cornell graduate Robert Morris. The "Morris" worm infected 6,000 mainframes, or about 10 per cent of the computers powering the internet at the time.
In 1999, David Smith of New Jersey wrote the Melissa virus, which spread via e-mail and infected Microsoft Word documents. Smith had two aliases - "Vicodin", when writing viruses, and "Doug Winterspoon", for when he was posing as a legitimate virus expert.
Filipino student Onel de Guzman released the "I love you" e-mail virus in 2000.
It tricked people into opening an infected e-mail attachment and installed a keystroke logger so he could get access passwords on infected machines.
Jan De Wit of the Netherlands wrote the Anna Kournikova virus in 2001 using the alias "On the fly". Created with virus-generation software, it tricked e-mail users into clicking on an attachment that purported to be a picture of the Russian tennis star.
