This is more commonly called a TSR, or Terminate and Stay Resident virus. Multipartite Multipartite viruses usually infect both boot records and files. Also called bimodal or bipartite. Overwriting Virus A virus that overwrites infected files. Sometimes used to refer to viruses that overwrite files with garbage data, effectively destroying the data overwritten.
Some viruses have a payload date, when they actually do something.
Payloads are like the clock ticking inside an infected a computer, a trigger date if you will, for when something will happen. A virus that has the ability to morph, or change, as it spreads. Polymorphic viruses are harder to identify since each copy of the virus is different. Thus, polymorphic literally means many shapes. The part of the virus that is in memory is called a resident extension.