A backdoor program is a Trojan specifically designed to allow malicious users to remotely manipulate affected systems. Like all Trojans, backdoors do not automatically propagate. They are either installed inadvertently by unsuspecting users or intentionally by malicious users.
Backdoors, like other Trojans, typically modify system settings to automatically start. Users may need to terminate backdoors before they can be deleted. Also, restoring affected systems may require procedures other than scanning with an antivirus program.