Macro viruses are always application-specific. Word Macro viruses will only infect Microsoft Word. A macro virus consists of a set of macros, nothing more. The macros must be stored wherever the application stores macros. In the case of Word, macros are stored in templates, rather than documents.
Word treats any file with the .DOT extension as a template, and any other extension as a document.
And such a template can be stored with any extension. So macro viruses for Word typically arrive at a computer as a template file with the .DOC extension. Others convert documents to templates with the .DOC extension, so that these files themselves are infected. Because users have no means of determining whether a file with a .DOC extension is a template with text or a true document, the virus can easily slip into a machine without user awareness. Your probability of getting a macro virus is directly proportional to the number of new, unknown documents you view each day.