With its hybrid nature, a multi-partite virus gains the worst qualities of each of its parents, and consequently is far more contagious and destructive than either. When opened, an infected document executes a macro automatically, or the user does so accidentally. The macro inflicts damage and then infects other documents on the disk. A macro is a set of executable commands designed to run in place of a repetetive task. Although macros are not unique to Microsoft products, it is through Microsoft products that the vast majority of macro viruses spread.
Macro viruses are quite common, but as a general rule, they do fairly little damage.
It is common, for example, for a virus to employ stealth and encryption. Stealth A stealth virus has many techniques at its disposal to help thwart detection. One technique is to redirect the addresses within a program that point to other programs or system information, so that they point to the virus file instead. When the program calls for that supplementary program or system information, it actually runs virus code. This infects the file without actually injecting additional code, which could show up as a symptom to virus scanning software.