Because of their similarity to viruses, worms also are often referred to as viruses. Trojan horses: A Trojan horse is a program that does something undocumented which the programmer intended, but that users would not accept if they knew about it.
A macro virus is thus a virus that exists as a macro attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to data files (that is, documents) rather than executable programs.
Computer viruses are never naturally occurring; they are always man-made. Once created and released, however, their spread is not directly under human control. Macro viruses: A macro is a piece of code that can be embedded in a data file. A macro virus is thus a virus that exists as a macro attached to a data file.
By some definitions, a virus is a particular case of a Trojan horse, namely, one which is able to spread to other programs (that is, it turns them into Trojans too). According to others, a virus that does not do any deliberate damage (other than merely replicating) is not a Trojan.
According to Webster's Collegiate Dictionary, a computer virus is "a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs or files, and that usually performs a malicious action (such as destroying data)". Computer viruses are never naturally occurring; they are always man-made. Once created and released, however, their spread is not directly under human control.