A macro virus is a virus that can be embedded in a program (such as word processor) exploiting the fact it allows macro programs. A well known one is the mellisa virus.
A macro virus can be spread through email attachments, discs, networks, modems, and the internet. Uninfected documents contain normal macros. Most macros start automatically when a document is opened or closed. A common way for a macro virus to infect a computer is by replacing normal macros with the virus. The macro virus replaces the regular commands with the same name and runs when the command is selected. In this case where the macro is run automatically, the macro is opened without the user knowing.
Once the application opens a file that contains a macro virus, the virus can infect the system. At the lease, it will begin to embed itself in other documents and templates, as well as future ones created. It may corrupt other parts of the system as well, depending on what resources a macro in this application can get access to. As the infected documents are shared with other users and systems, the virus will spread.
