A very similar example demonstrates that there are viruses for which no error-free detection
algorithm exists. That is, not only can we not write a program that detects all viruses known and
unknown with no false positives, but in addition there are some viruses for which, even when we
have a sample of the virus in hand and have analyzed it completely, we cannot write a program
that detects just that particular virus with no false positives.2 As noted above, a virus is said to be "polymorphic" if the size of the viral set is greater than one; that is, if the code of the virus is different in different infected objects. Consider a virus which is sufficiently polymorphic that for any implementable algorithm X the program p:
