I agree with you in large part, however, we do not have perfect information with respect to viruses in terms of their history of changing or not changing modes of infection. Since all viruses presumably evolved from a single virus, we should probably assume that such a change has taken place – but over millenia, not in a few years.[/quote]
We don’t have perfect information on the history of viruses, no. But even if all viruses evolved from a single virus, that doesn’t necessarily mean that any virus has ever changed its mode of transmission among mammals. Mammals have been around for a couple hundred million years; viruses have been around a lot longer. Maybe they have changed method of transmission at some point over those two hundred million years. Maybe not. Humans have only been around a couple hundred thousand or a couple million years, depending on how you define them. There’s a good chance no virus has ever changed its method of transmission among humans.
