Thus, polymorphic literally means many shapes. The part of the virus that is in memory is called a resident extension. Retrovirus Also called a anti-antivirus virus, this is a virus that attacks, disables, or avoids infecting specific antivirus software. It's just one of many methods used by virus authors to avoid detection by antivirus software. Any kind of malicious type program, Trojan Horses, viruses, etc, are considered to be rogue programs.
Some viruses use self-encrypting techniques to try to hide their presence.
Self-encryption often makes them look more like data within a file rather than the pattern of a virus. This is a stealth technique. Signature Each virus has it's own specific pattern, ways of infecting, etc. A series of letters and numbers within the code of a virus are often unique, providing antivirus developers with a signature of the virus. It's like a thumbprint for the virus, giving antivirus developers the ability to identify the virus based upon a unique string.