One way of covertly introducing viruses into a computer is through the use of a MACRO. A macro consists of several instructions grouped together which then carry out a series of instructions. Macros are normally very useful tools, that make working with computer programs easier.
Common examples macro functions are: automatically inserting the date and time into your document, displaying a picture in your document, adding a sound to your document, or automatically calculating something in a spreadsheet. To perform these functions, the computer has to follow quite a set of complicated instructions. These are called macros.
Sadly, macros can also be used to cause harm to your computer or files. When a macro is used this way it is called a virus. When a macro is inserted to cause harm, it might not even be visible; or might be disguised as something desirable such as an innocent picture.
Far from all viruses in the form of macros. Some viruses come in the form of program files, where the virus file is made to look like part of a computer program. Other Viruses are disguised as part of the operating system (the operating system is the main program that runs your computer, such as Windows XP, Windows 2000, etc.) This, of course, makes it hard for someone who is not a computer technician to detect and fix them. And even computer technicians can be hard put to find and handle them. That is why we use antivirus programs.