There is an informational entity known as the metavirus, which causes information systems to infect themselves with customized viruses. This may be just a basic principle of nature, like Darwinian selection, or it may be an actual piece of information that floats around the universe on comets and radio waves -- I'm not sure. In any case, what it comes down to is this: Any information system of sufficient complexity will inevitably become infected with viruses -- viruses generated from within itself.