Why did the Roman Empire, which dominated Europe and the Mediterranean for five centuries, inexorably weaken until it disappeared? Archaeologists, specialists in ancient pathologies and climate historians are now accumulating clues converging on the same factors: a powerful cooling and pandemics. A disease, whose symptoms described by the Greek physician Galen are reminiscent of those of smallpox, struck Rome in 167, soon devastating its army. At the same time, a sudden climatic disorder that was underway as far as Eurasia caused agricultural yields to plummet and led to the westward migration of the Huns. Plagued by economic and military difficulties, attacked from all sides by barbarian tribes, the Roman edifice gradually cracked.