The ILOVEYOU worm, also known as VBS/Loveletter and Love Bug worm, is a computer worm written in VBScript.

Description
The worm, first discovered in Hong Kong, first arrived in e-mail boxes on May 3, 2000 with the simple subject of "ILOVEYOU" with an attachment "LOVE-LETTER-FOR-YOU.TXT.vbs".

Two aspects of the worm made it effective:
It employed a mechanism — VBScripts — that, while not entirely novel, had not been exploited to a degree previously to direct attention to their potential, reducing the layers of protection that would have to be navigated for success.
It used a powerful psychological motivation to entice users to open the e-mail and ensure its continued propagation.

Spread

Its massive spread moved Westward as workers arrived at their offices and encountered messages generated by people from the East. Because the virus used mailing lists as its source of targets, the messages often appeared to come from an acquaintance and so might be considered "safe", providing further incentive to open them. All it took was a few users at each site to access the VBS attachment to generate the thousands and thousands of e-mails that would cripple e-mail systems under their weight, not to mention overwrite thousands of files on workstations and accessible servers.

Effects

This particular malware caused widespread outage for an estimated $5.5 billion in damage, making it the most damaging worm ever. The worm overwrote important files, as well as music, multimedia and more, with a copy of itself. It also sent the worm to everyone on a user's contact list. This particular worm only affected computers running the Microsoft Windows operating system.

from : en.wikipedia.org