The Black Death
Carried west along the very trade routes the Mongols had opened, bubonic plague reached Europe and killed perhaps a third to a half of everyone on the continent in just a few years. The horror was total — but for the survivors, suddenly scarce, it inverted the old order: labor could now demand high wages, walk away from serfdom, and claim rights it had never held. The catastrophe compressed concentrated wealth and rewrote the bargaining power of ordinary people, who had held almost none of it before.
A pathogen spreads through a population and reshapes its demography, economy, and beliefs at once.