A polymorphic virus creates an polymorphic infection in a computer. It is designed to create copies of itself, changing the arrangement and byte values of its contents each time, with the purpose of evading detection by virus detection software. It may also encrypt its contents in such a way that antivirus software cannot positively identify and remove it.
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.