zlacker

If you are trying to do a bayesian analysis, you need a prior probability. That is, what it would be expected to be, without your analysis. This would be zoonosis.

There is never absence of evidence for zoonosis, because by association all other viruses came to be via zoonosis. So, before you do any further investigation, it's the default. And then, when you do, zoonosis is already the null hypothesis as it was the most likely before you being your further investigation.