In May 2000, a new virus ripped through the world in record time, infecting more than 50 million computers in the first two weeks. The virus spread via e-mail, using a combination of social engineering, a barely known but powerful Windows scripting engine, and the Windows setting that hid file extensions by default.
The virus came as an attachment from two young computer programmers in the Philippines, with an e-mail stating "ILOVEYOU" and LOVE-LETTER-FOR-YOU.TXT. In fact, the attached file was actually a VBS script, but because of the way Windows processes filenames when the computer is set to hide the extensions, it did not end up with the full .txt.VBS file name displayed. Only if you had changed the default settings in Windows so that file extensions would always be displayed, you would see what kind of file the love letter really was.
Once clicked, the ILOVEYOU virus used a very powerful script engine that was hidden in Windows very few people were aware of it. Many recipients, who would hesitate to open a foreign file, considered it a harmless text file and opened it. As the virus spread using the address book of the previously infected host, the love letter came from a person you actually knew-the temptation to open and read a love letter from someone you had a personal relationship with and to you was addressed was simply too strong. Hundreds of thousands of people opened the e-mail.
It was a perfect storm: the end users' deep curiosity about the love letter in combination with the powerful and vulnerable scripting engine. The virus swept west around the world as offices in each time zone opened for business. Big companies and government departments had to shut down their mail servers just to protect themselves. Total damage worldwide was estimated at $ 5.5-8.7 billion and an additional $ 1
Unlike other computer worms that self-replicate, the ILOVEYOU virus replicates itself by overwriting files with JPG, JPEG, VBS, VBE, JS, JSE, CSS, WSH, SCT , DOC, HTA, MP2 and MP3 extensions on the host computer and replace them with copies of themselves by appending the additional VBS file extension and no longer booting the user's computer. Until the rapid spread of the Mydoom virus in 2004, ILOVEYOU set the record for fast-paced email-propagated virus in history.
Courtesy of the Computer Virus Wiki.