A polymorphic virus creates an infection in a computer that is known as a polymorphic infection that creates copies of itself, with each copy different to fool a virus detection and users. The variations are typically different forms of encryption or other signatures to make it more difficult for a virus detection program to find and remove the virus from a computer.
Because one polymorphic virus could have hundreds or thousands of variants it makes it more difficult to detect every variant of the virus. Developers that design the detection programs have to write extra lines of code to make the programs better at detecting the virus infections. Even the best antivirus programs can have trouble with detecting and cleaning polymorphic infections or not giving false positives. However, antivirus programs with heuristic detection do have a better time at detecting these types of viruses.