Cohen and his supervisor Adleman defined a virus as follows: "A virus is a program that is able to infect other programs by modifying them to include a possibly evolved copy to itself". This definition seems to be well accepted by the computer security community as a foundational definition. Thus, a virus is a self-replicating program, whose offspring may be a mutation of the original program. Viruses thrive in our computers, which are based on Turing's model of computation. We discuss the fundamental reasons for this.