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.
