A Bayesian prior of 50/50 seems high to me. It assumes that 50% of new disease variants come from lab leaks.
In the last few decades there have been 1-2 confirmed lab leaks per year. And they're often thing like "we found a vial of smallpox we didn't know we had" not new diseases.
Nature very capably produced colds, flus, a bunch of nasty diarrhoeal diseases, the many and varied sexually transmitted diseases, the hemorrhagic fevers, and so on. For "some new disease variant that I don't know anything about", my prior would be more like 1/99 lab leak to natural origin.
In the last few decades there have been 1-2 confirmed lab leaks per year. And they're often thing like "we found a vial of smallpox we didn't know we had" not new diseases.
Nature very capably produced colds, flus, a bunch of nasty diarrhoeal diseases, the many and varied sexually transmitted diseases, the hemorrhagic fevers, and so on. For "some new disease variant that I don't know anything about", my prior would be more like 1/99 lab leak to natural origin.