Unlike typical file infecting viruses, macro viruses infect data files - chiefly files created in Word, Excel, PowerPoint, or even Access. Visual Basic macros are actually mini-programs embedded in the document, and thus have many of the same rights and abilities as the user who is logged on to the system.
In Microsoft Office, macro viruses generally spread by first infecting the global template, (i.e. Normal.dot in Word), in turn infecting other documents as they are accessed. Exact methods can vary; for example, some macro viruses only infect currently open documents.
As with any other malware, effects from a macro virus infection can range from benign annoyance to loss of critical data.
In the late 1990s and early part of 2000, macro viruses were one of the more commonly encountered forms of infection. Though Microsoft has made several improvements designed to hamper the spread of macro viruses, macro viruses still continue to circulate to some extent.
Because macro viruses infect data files and not program files, Macintosh users are generally susceptible to macro viruses as well.