It's a simple explanation: Diseases live on by finding victims to infect, and diseases that jump species (sometimes undergoing changes in the process) find a whole new set of victims available to them. Yet this is too pat an explanation for many of us, and we tend to reject that which doesn't fit well to our understanding of our world, substituting in its place myths that, although disturbing, at least seem more logical. We make the arrogant assumption that our world should always make sense according to our current level of knowledge; such a premise works well in most cases but breaks down in the face of matters our understanding does not yet encompass. At those moments at least some of us scramble to find explanations -- any explanations -- that can be shoehorned into service.
