I am still waiting for the new conventional wisdom about what is happening to emerge, and I believe it will be as follows.
A particular ancestral betacoronavirus emerged in bats several decades ago with a special superpower, different from but conceptually not too distinct from HIV’s ability to rapidly mutate. This virus had the ability to easily spread out among many animal species and evolve among them through a standard slow process of mutation subject to selection pressures, but then to occasionally co-infect a single host and recombine to create a radically different variant (a “chimera,” although I think it’s better thought of as an “offspring.”). These offspring would occasionally be very deadly because they combined well-developed abilities that had evolved in separate lineages from the original ancestor evolving in separate species.
Eventually I think we will categorize all the recent betacoronavirus outbreaks (Sars-1, Sars-2, MERS) as part of this broader process, and require a vaccination strategy that can be quickly deployed against new recombinations from this original ancestral betacoronavirus as they randomly emerge from the primordial stew across many animal species, including ours. The evidence thus far points to recombinations resulting in the emergence of a distinct dangerous variant with some regularity.
This story also explains the existence of some preexisting immunity in much of the population to Sars-Cov-2, but substantial variation in what feature of Sars-Cov-2’s genetic code the immune system reacts to depending on whether the individual is known to have had SARS, MERS, or neither. In all likelihood, possibly many relatively nonlethal or even asymptomatic variants of the same betacoronavirus ancestor have been circulating undetected among human populations during this same 10-20 years, resulting in people people who have been exposed to different random bits of genetic material present in Sars-Cov-2.
Here is the link. By the way, it turns out that smallpox is much older than we had thought (NYT). Betting on origins being longer and deeper than other people expect is often the bet to make.