I’m not a biologist but as I understand, viruses very rarely go from being common in one species to another. It has to do with mutation and how you want many hosts with high viral load to stumble on an effective change.
There are cases where someone close to animals might get it, a farmer typically, but it rarely becomes transmissible between humans — not unless it mutated first and for that it would need to be endemic among farmers. When we observe cases among relatives of the farmers who are not exposed, that’s when we panic. Swine flu and bird flu would fall in that case: there had been many cases among farmers, for decades, and they learned to dismiss them (making cases harder to track). When relatives got sick, epidemiology authorities were suddenly a lot more worried, to the puzzlement of farmers who had been sick prior.
More so, viruses are endemic in bats (who have notoriously poor immunity) but most of them can’t be transmitted to an animal with high immunity, like humans; not without going through mutation and selection through another animal with a more selective immunity, hence the idea of an intermediary host. The pangolin was suggested for a while, if you remember. There was a lot of talk about which animal, but what those stories didn’t really clarify is the “the” in “the pangolin”: whether virologists were expected:
1. widespread infection among pangolins living together; the meaning the species — but then we would have noticed entire groups being sick; or
2. if one single animal was infected by a bat, and someone was unlucky enough to be host to a human-compatible mutation (very unlikely) and then that human was host to another mutation that made it human-to-human-transmissible (also unlikely).
A lot of the early debate on Covid was surprised at how it seemed to have skipped that middle step, or how we couldn’t find it. If you are more familiar with physics, that’s a bit like saying you saw the tunnel effect at the human scale. Sure, we know it’s possible in theory, but… so unlikely, we would probably need to change the science to account for it anyway. Physicists would naturally side with whomever is trying to debunk the observation. That’s how science works: the burden of the proof is on who is making an unlikely claim given current model. The natural history explanation of Covid is possible, but extremely unlikely given “species barrier”.
Once again: I’m not a biologist, but a lot of the scenario that would make that possible would change our approach to epidemiology:
1. if the virus was undetected among humans close to bats for a while, and mutated to become transmissible, we’d need to think of monitoring asymptomatic humans;
2. if the virus was present and undetected in an intermediary species, that’s more likely but still very worrisome and would require significant changes to veterinary practices;
3. if there was a genetic manipulation towards gain of function, the political consequences would be a shit-show but it would preserve our understanding of how frequently and consistently viruses mutate.
In that particular case, the instinct of scientists was split, between people who see the gains-of-function research valuable and those who defend the model — which is why there isn’t a clear view of what “scientists” think. However, don’t let that disagreement confuse you: clarifying what happened will have scientific impact, be it ban a controversial approach, or change our understanding of inter-species viral transmission.