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Most modern anti-virus programs try to find virus-patterns inside ordinary programs by scanning them for virus signatures. A signature is a characteristic byte-pattern that is part of a certain virus or family of viruses. If a virus scanner finds such a pattern in a file, it notifies the user that the file is infected. The user can then delete, or (in some cases) clean or heal the infected file. Some viruses employ techniques that make detection by means of signatures difficult or impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus.
•In the past, some viruses modified themselves only in simple ways. For example, they regularly exchanged subroutines in their code. This poses no problems to a somewhat advanced virus scanner.