A type of computer virus that differs from a regular virus in the way it disguises itself. Polymorphic malware are capable of changing their program code with each new copy by means of encryption. Thus each new copy of the virus is a new copy different from the previous one, making it much harder to detect by standard antivirus programs that use signature databases. Detecting polymorphic viruses and treating the infected system requires complete decryption of the malicious code.
Traditionally, polymorphic viruses are classified according to their polymorphism levels, which can range from 4 to 6. The simplest viruses have the same parts of the malicious code, by which they can be detected by antivirus programs. In the most complex version, a polymorphic virus is capable to completely update its source code each time it is copied.