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 can thus develop appropriate anti-virus software. Thefirst polymorphic viruses are known from the 1980's. In
addition to polymorphic viruses, there is another type of virus, the metamorphic virus, which changes its external form, but its functionality remains the same.