> if the source was natural and zootonic, we would have expected to find the intermediate animal by now. Only if you assume it's both widespread and endemic in

> if the source was natural and zootonic, we would have expected to find the intermediate animal by now. Only if you assume it's both widespread and endemic in the intermediate animal population. If it's spreading at low levels through that population for some reason (like being a less social animal) then it's trying to find a needle in a haystack, and that's on top of the needle/haystack of identifying the correct animal and population. If it's not endemic we may never find it, perhaps the intermediate population were well on their way to herd immunity before the virus jumped to humans leaving little to no trace. There are some other possibilities too, like it could have been transmitting undetected among relatively isolated humans (like small farming villages) for some time before an infected farmer visited a market. This would significantly broaden the scope of when and where the infected animal population was. That we haven't found the source yet is incredibly weak evidence.