It's one of those things that keep being flogged because people want someone to blame. We have trouble accepting that it is most likely the result of our increasing incursion into wild spaces. It took more than a decade to find the source of original SARS. MERS was much easier only because it was in camels and that was one of the first places researchers looked.

But people are impatient and so come up with this emotionally satisfying theory then demand that people who doubt it prove a negative. It is rather exhausting.