When it began: In 1989, a diskette proclaiming to be a database of AIDS information was mailed to thousands of AIDS researchers and subscribers to a U.K. computer magazine. The diskettes contained Trojan software that rendered the computers useless and demanded that $378 be sent to PC Cyborg Corporation at a post office box in Panama. The software was linked to an American doctor and AIDS researcher named Joseph Papp, who successfully invoked the insanity defense when he was extradited to the U.K. in 1990. To see an explanation of the Trojan horse exploit via diskette, see this article at Purdue University.

What it is: Trojan horse software installs itself on the user's computer when they click on a link or a disguised computer file or attachment. Once installed, the software can be controlled remotely by hackers to extract money, passwords and other sensitive information. It can also be used to create a relay point, or zombie, for forwarding advertising spam, phishing e-mails and Trojan software to millions of other computers on the Internet.

Response: IP address blacklists; Bayesian content filters; content heuristics engines; content fingerprinting schemes augmented by sender authentication; anti-virus software; network monitoring; teaching computer users not to click on communications or software that they are not expecting to receive.