You create a mutating virus. The virus in itself includes a mutating compression algorithm that uses the MD5 sum of the mutating virus as the basis of the mutations of the compression algorithm. Given the compression will change each time the virus mutates, the virus definitions must be very generic* which would lend itself to many false-positives, unless you have an excellent heuristics engine (Avira has one of the best out there).
*This is speculation. I have only written small virus definitions, nothing to the extent of a mutating virus.