Another theory is that the original covid was both less virulent and more lethal, but the US spread our own strain which was less lethal and more virulent... i.e. a vaccine.

It explains a lot of irregularities, such as why it affects different people in such drastically different ways. Sometimes seems to spread easily, and other-times not as easily as you'd think. So much is explained by the idea that there have been both a dangerous strain and an innocuous strain since very early on.