You also use Word 6. Thereafter, any document that you create will contain the virus. These documents can spread the virus to other users of Word 6 or later if they open the document on their machine. Just as you can't run a Macintosh program on a PC etc., most viruses only affect one type of computer. That is, Macs typically can't get PC viruses, and vice versa.
Macro viruses are a new phenomenon.
For example, you might have a macro in your word processor that sets the default font to Times 12, adjusts the line spacing, runs spellcheck, and saves the file. You could perform all of these steps automatically by using the macro. The problem started with recent versions of Microsoft Word and Excel. The macro language in these products is very powerful, and in fact is a kind of programming language. This macro programming language is powerful enough to allow the creation of viruses.