[Adleman90] L. Adleman, An Abstract Theory of Computer Viruses, Lecture Notes in Computer Science, V403, Advances in Computing - Proceedings of Crypto-88, S. Goldwasser, Ed., Springer-Verlag, 1990. [This was the second theoretical work on computer viruses, describes a subset of the previous formal definition, and demonstrates that detection of members of this subset is also undecidable.]