A macro virus is a malicious program that is written by using the macro language built into a software application such as Word. Since Microsoft Office allow macro programs to be embedded within documents, so that the programs may be run automatically when the document is opened, this provides macro viruses with a distinct mechanism to spread. The virus can infect other Word documents and propagate itself among data files and can even harm your computer's operating system.
A well known example of a macro virus is the Melissa Virus from 1999. The virus would infect a system through Word documents and then send itself by email to the first 50 people in the person’s address book. Since a macro virus depends on the application rather than the operating system, it can infect a non-Windows computer such as a Mac.
The first defense against any viruses and malicious program is your anti-virus software. But macro viruses are known to quietly pass through many security software. Fortunately, newer versions of Microsoft Office prevents macros from running when a user opens a Word document with a macro. Instead, users are presented with a warning and an option to open the document with the macro enabled. You should not open a document with the macros active, unless you are absolutely sure that the document contains no harmful macro viruses.