A macro is an instruction that carries out program commands automatically. Many common applications (e.g. word processing, spreadsheet, and slide presentation applications) make use of macros. Macro viruses are macros that self-replicate. If a user accesses a document containing a viral macro and unwittingly executes this macro virus, it can then copy itself into that application's startup files. The computer is now infected--a copy of the macro virus resides on the machine.
Any document on that machine that uses the same application can then become infected. If the infected computer is on a network, the infection is likely to spread rapidly to other machines on the network. Moreover, if a copy of an infected file is passed to anyone else (for example, by email or floppy disk), the virus can spread to the recipient's computer. This process of infection will end only when the virus is noticed and all viral macros are eradicated. Macro viruses are the most common type of viruses. Many popular modern applications allow macros. Macro viruses can be written with very little specialist knowledge, and these viruses can spread to any platform on which the application is running. However, the main reason for their 'success' is that documents are exchanged far more frequently than executables or disks, a direct result of email's popularity and web use.