A polymorphic virus creates an infection in a computer that is known as a polymorphic infection. A polymorphic virus is one that creates copies of itself, with variations in each copy to fool a virus detection program and user. The variations are typically different encryption methods in the virus file copies, which makes it more difficult for a virus detection program to detect and remove a polymorphic virus from a computer.
Polymorphic infections are difficult for virus detection programs to cleanse because one polymorphic virus could have hundreds or thousands of variants. Developers that design the detection programs have to write extra lines of code in order to make the programs better at detecting the virus infections. Even the best antivirus programs have trouble with detecting and cleansing polymorphic infections, although antivirus programs with heuristic do have a better time at detecting these types of viruses.