In reality it is a highly dangerous backdoor designed by a cracking group called the Cult of the Dead Cow Communications. It is usually distributed by malicious people in the form of a Trojan Horse attack . During installation, it does not give any indication of what is really going on. Once installed, the server is intentionally difficult to detect on your machine, yet allows almost complete control over your computer by the remote attacker. Back Orifice is not a virus.
Viruses reproduce on their own.
The Back Orifice server has to be willingly accepted and run by its host before it can be used. However it is usually distributed claiming to be something else. It could be considered a trojan horse. In the case where a user accepts a program, and runs the program without understanding what it is. People run it and nothing appears to happen so they ignore it, the server deletes itself as well after running.