We’ve already mentioned that some applications, like word processors and spreadsheets, can be made to change the way they work by means of programs called Macros. Unfortunately these macros, as well as doing useful things like automatically formatting a document, can be used to attack your computer.
A macro virus infects an application like Microsoft Word and causes a sequence of actions to happen when the application is opened. One common effect is the insertion of joke text into a document. Macro viruses are often spread via e-mail. Well-known examples include the Melissa virus and the Nimda worm which attacked systems in many different ways, including via e-mail and back doors.
Because macros are stored in documents they are sometimes seen as less of a threat than other programs, but this is not really the case as they can do serious damage. You should never open documents if you are unsure of where they come from. Opening a document allows any macro viruses it contains to run and your computer can become infected.
Next: Resident Viruses