Programs that reproduce and possibly evolve. Examples include
the 11,000 or so known viruses, custom file viruses designed to act against
specific targets, and process viruses that cause denial of service or
thrashing within a single system.
Complexity: Virus detection has been
proven to be undecidable in the general case.
[Cohen86][Cohen84] Viruses are also trivial to write and highly effective
against most modern systems.