Polymorphic or mutation viruses
Polymorphic (mutation) viruses are unique in that they are designed to elude detection by changing their structure after each execution--with some polymorphic viruses, millions of permutations are possible. Of course, this makes it harder for normal antivirus programs to detect or intercept them. It should be noted that polymorphic viruses do not, strictly speaking, constitute a separate category of virus; they usually belong to one of the categories described above.