A macro virus is a computer virus written in the same macro language used for software applications like word processors.  Its effect is to release a chain of events in conjunction with the application.  Microsoft Word is an example of an application susceptible to macro viruses; this explains why it is a bad idea to open suspicious or unknown attachments even if they may appear legitimate.  Because macro programs embedded in these documents run automatically when the document is opened, it is a likely mechanism to spread viruses.  Once triggered, the macro virus can embed itself in other documents, including any future documents created after the virus attack, as well as conceivably download software to the target computer.  An example of this type of virus is the Melissa Virus from 1999; after opening the infected document from email, the targeted computer would then send itself to the first 50 email addresses in the person’s contact list, thereby replicating itself quickly.  Because a macro virus works using the application rather than an operating system, it can also infect non-Windows computers as well.  Macro viruses are also known as script viruses and can also be embedded within web pages.  The best defense against being infected by a macro virus, besides being very careful of what email attachments you open, is having a quality, updated antivirus program.