Historians have attributed the decline and fall of the Roman Empire to corruption, Christianity, barbarian invasion, and hundreds of other factors. Harper offers a striking reinterpretation with worrisome implications for the present day. Ancient Rome, he argues, succumbed to pandemic diseases. He documents the devastating spread of plague, tuberculosis, smallpox, and perhaps even Ebola in ancient Rome, which together killed more than half the population in some areas. He shows how the thousand-year-old empire grew vulnerable to such pathogens owing to severe Mediterranean climate change, extremely large cities, a vulnerable food chain, and its central position in vast networks of travel, trade, and migration. In making the case, Harper relies in part on fascinating new biomedical tools that are revolutionizing what is termed “paleomolecular archaeology.” Yet due to the fragility of many pathogens, his study also relies on the subtle interpretation of traditional written sources, including many religious texts. The lesson is clear. Today, we inhabit a global system with a very similar combination of climatologic disturbances, urbanization, less diverse diets, and globalization. Ancient history reveals the risks we run.