The Trojan Horse programs and computer viruses in the real world, cannot be treated as the realizations of turing machines. In this article, the actions of Trojan Horses and viruses in the real world are considered and a minimal framework is described for their modelling for a better understanding. biological metaphors and some similar conventional approaches are shown to be inadequate. Some suggestions for constructing virally-resistant systems are also put forward.
Computer viruses also mimic the behaviour of biological pests. The facilities provided by the host computer are hijacked and they cross-infect the other hosts as well. Various forms of damage are caused when these viruses are activated. The computer viruses are now widespread. there are even virus construction kits that anyone can use to develop sophisticated viruses. The hackers write virus programs in plenty. The standard models of computation, such as Turing Machines are first taken as the starting points of the investigation. But the systems that interact cannot be precisely modelled by the Turing Machine. The computational process undertaken by a virus does not terminate. Viruses are a menace for any kind of systems and particularly the ones connected in networks.

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