Definition from wikipedia.org
In computer security technology, a virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents (for a complete definition: see below).A computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells. Extending the analogy, the insertion of the virus into a program is termed infection, and the infected file (or executable code that is not part of a file) is called a host. Viruses are one of the several types of malware or malicious software. In common parlance, the term virus is often extended to refer to computer worms and other sorts of malware.
Since the mid-1990s, macro viruses have become common. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Outlook. These viruses spread in the Microsoft Windows monoculture by infecting documents and sending infected e-mail. Some versions of Word have had bugs in the calls by which macros replicate themselves, causing occasional replication errors, which has sometimes resulted in actual evolution by natural selection.
The Melissa virus was released on 26 March 1999 and was designed to infect macros in wordprocessing documents used by the Microsoft Word 97 and Word 2000 programs.
The innovative feature of the Melissa virus was that it propagated by e-mailing itself to the first fifty addresses in the Microsoft Outlook e-mail program's address book. This feature allowed the Melissa virus to propagate faster than any previous virus. The virus arrived at each new victim's computer disguised as e-mail from someone whom they knew, and presumably trusted.
Norton Anti-Virus calls it VBS.Loveletter.A. The ILOVEYOU worm arrived at the victim's computer in the form of e-mail with the ILOVEYOU subject line and an attachment. The e-mail itself was innocuous, but when the user clicked on the attachment to read the alleged love letter, LOVE-LETTER-FOR-YOU.TXT.VBS, the attachment was a Visual Basic program that performed a horrible sequence of bad things.
This included deletion of files from victim's hard disk and password theft.