Document or “macro” viruses take advantage of macros – commands that are embedded in files and run automatically.

Many applications, such as word processing and spreadsheet programs, use macros. A macro virus is a macro program that can copy itself and spread from one file to another. If you open a file that contains a macro virus, the virus copies itself into the application’s startup files. The computer is now infected.

When you next open a file using the same application, the virus infects that file. If your computer is on a network, the infection can spread rapidly: when you send an infected file to someone else, they can become infected too. A malicious macro can also make changes to your documents or settings.

Macro viruses infect files used in most offices and some can infect several file types, such as Word and Excel files. They can also spread to any platform on which their host application runs.

Macro viruses first appeared in the mid-1990s and rapidly became the most serious virus threat of that time. Few viruses of this type are seen now.