When it began: In 1988, Robert T. Morris, Jr., a graduate student at Cornell University and son of a National Security Agency scientist created software that would automatically replicate itself on computers hooked up to the government's ARPAnet (the precursor to the Internet). Though Morris, who is now a professor at MIT, insisted he was only trying to gauge the size of the Internet, the worm may have infected thousands of government computers and caused anywhere from $10-$100 million in damage, according to the U.S. General Accounting Office.

Morris was convicted of violating the 1986 Computer Fraud and Abuse Act and was sentenced to three years' probation, 400 hours of community service, and a fine of $10,050. The worm shattered perceptions of the emerging Internet's security and stability.

What it is: A more dangerous evolution of viruses, worms are self-propagating, meaning they do not need any intervention from the victim—such as clicking on an infected software attachment in an email—to transfer themselves to other computers. Instead, they rely on vulnerabilities in software and networks to allow propagation. For example, the "security patches" offered by Microsoft and other software vendors are usually the result of the discovery of a vulnerability in the software that could allow hackers to take control of users' computers without warning.

Response: Anti-virus software, network monitoring, security training.