Fred Cohen in 1986 has proved in his Ph.D. thesis the undecidability of viral detection (no perfect virus checker can ever exist).
Does this imply that there exists a totally undetectable virus?
Else is it possible for a malware to stay a very long time (lets say many years or forever) undetected for the antivirus industry (COTS products)?
Furthermore is it possible for a malware to stay undetected for a very long time from defence agencies?
What are the best ways to approach this goal (perhaps make the behavior of the program chaotic, unpredictable or add time complexity as in cryptographic algorithms, use a rare or common host program...)?