Polymorphic virus

Virus that re-encrypts itself with each infection

A polymorphic virus is one that encrypts its code differently with each infection, or generation of infections.

The aim of this behaviour is to make it difficult for anti-malware software to detect all files infected with the virus, requiring much more sophisticated detection techniques than simple file-infecting viruses, which insert their code unchanged into each infected file.

The same term is often also used to cover metamorphic viruses.