Alan Cox talks about the next 50 years of computer security. Early in the article he makes the comment (which is a commonly held view),
Alan Cox: It is beginning to improve, but at the moment computer security is rather basic and mostly reactive. Systems fail absolutely rather than degrade. We are still in a world where an attack like the slammer worm combined with a PC BIOS eraser or disk locking tool could wipe out half the PCs exposed to the internet in a few hours. In a sense we are fortunate that most attackers want to control and use systems they attack rather than destroy them.
This is not strictly true. In nature, the most virulent pathogens in nature keep their hosts alive for long enough to pass on the disease to as many other hosts as possible. Reproduction, not death is the goal here. Any (computer) virus that killed its host immediately would fail to propagate effectively and time delay self destructs tend to be disarmed by the anti-virus crew.