A macro virus is a virus that propagates through only one type of program, usually either Microsoft Word or Microsoft Excel. It can do this because these types of programs contain auto open macros, which automatically run when you open a document or a spreadsheet. Along with infecting auto open macros, the macro virus infects the global macro template, which is executed anytime you run the program. Thus, once your global macro template is infected, any file you open after that becomes infected and the virus spreads.