The Antonine Plague
At the height of Rome's power, soldiers returning from a war in the east carried home a devastating plague — probably smallpox — that swept the empire and killed millions, likely including an emperor. It thinned the army and the tax base at the worst possible moment, marking the point where Rome's long confidence began to curdle into strain. Disease had struck a superpower not at its edges but through its manpower and its tax base, the two things holding it together.
A pathogen spreads through a population and reshapes its demography, economy, and beliefs at once.