Macro Viruses and the Melissa virus
Microsoft thought it was doing it's customers a favor by adding a programming language to Microsoft Word. In terms of customer service, it was a great idea, because it would allow users to automate and program within their documents. For example, when a document opened, it could be programmed to ask the user for details that must be entered into each document, like insurance policy numbers or phone numbers.
Microsoft didn't count on this programming language ever being used to turn Word documents into virus infectors, but that's exactly what happened.
The first Macro Virus was called the Concept virus. It was designed in 1995 simply to show that it was possible to write a virus in Word's Macro language. Once it was proven, though, the idea took off. By 2004, nearly 75% of all viruses were macro viruses.
When Word opens a document, it runs a normal series of macros. When the system is infected, these normal macros have been replaced, so that when any future documents are opened, their macros are infected as well. Every Word document this computer touches carries a copy of the virus, and will infect any other system that opens it.
Possibly the most famous macro virus to date was Melissa. Virus programmer David L. Smith named the code after a lap dancer he knew, and released it in late March, 1999. The virus sent a file called 'List.doc' which it claimed were passwords to eighty adult websites. Anyone who opened the document would get their passwords and a free copy of the Melissa macro virus.
Melissa would then gather up the first fifty entries in the address book, and email itself to all of them. Melissa had infected so many systems that by March 26th, it was shutting down mail servers with all of the infected emails traveling across the 'net.