this is how a virus can be of innocuous character then suddenly shift to something that is quite problematic. this can occur spontaneously as per wobbly fit and frameshift, >or< it [frameshift] can be anticipated and a sequence engineered that will be of no concern for most of the time until a frameshift occurs by chance and results in a product that has lethal character.

https://en.wikipedia.org/wiki/Frameshift_mutation

https://en.wikipedia.org/wiki/Open_reading_frame

this can be quite natural, and it can also be an insidious method of engineering a virus that starts out as nothing of obvious concern but becomes a weapon by chance after a sufficient number of reproductive events.

The question one must ask: where were the chances of zoonosis greatest (where did people have the most contact with relevant animals)? and everything else falls into place