polymorphic virus
Viruses work according to certain code sequences and can be recognized by recurring byte sequences. However, there are also viruses that constantly change their own program code by creating copies of themselves that have been modified by permutation.
Such viruses are called polymorphic viruses. They can work with changing signatures, but they are subject to certain algorithms. The manufacturers of virus scanners and anti-virus software analyze this set of rules and are thus able to develop appropriate anti-virus software. The first polymorphic viruses are known from the 1980s.
In addition to polymorphic viruses, there is another type of virus, metamorphic viruses, which change their external form, but their functionality remains the same.