Macro viruses infect data files, rather than executable files. Data files are exchanged far more frequently than executable files.
Macro viruses are capable of propagation across different machine types and operating systems.
Microsoft Word viruses will work in Word for Windows 3.x, Windows 95, Windows NT and Word for the Macintosh. Applications such as Microsoft Word, Microsoft Excel, etc. allow you to create macros. Viral code residing within a macro will be executed every time the macro is accessed. If a document containing viral code is opened, the macro virus will be executed. After this, every document created or modified becomes infected by the virus.