Oops! 2/3 had,1/3 died,2/3 left
Yee—aahh, pretty much what I said. And somehow different conclusions. Mmkay.
You see it as proof that something evolved. I see it as proof that something already existed and had no need to evolve.
Like, I dunno, People. Or tigers.
The earlier theory was that the disease mutated as it moved North and West and eventually became non-infectious.