The Animal Episode is a very interesting description of how one of the first computer viruses was written. ANIMAL was a game which would attempt to guess the animal that you were thinking of; if it couldn’t guess, it would add your animal to it’s tree so that it would do better next time. The creator, John Walker, also wrote a program called PERVADE which would try to copy ANIMAL and itself to all the available directories in the system – essentially the behaviour of a computer virus.
The especially geeky people out there may be interested in the assembler code provided, which gives a bit of an insight into what coding was like back in the good old days!