Polymorphic viruses will alter their codes to avoid being detected by anti-virus programs. These viruses encrypt themselves differently every time they infect, making it harder to track and prevent them. To prevent attacks by polymorphic viruses, specific signature codes are being developed to search for the different varieties within one type of virus.
A virus that, when replicating itself, creates copies that are different from itself, making them harder to detect. Each copy may use a different encryption algorithm, so each will look entirely different from the original and from each other. This property makes it harder for anti-virus software to recognize each strain of the virus, so the odds are higher that some strains will get through. See Also: Mutating Virus.