Despite advances in antibiotics and vaccines, and the successful eradication of smallpox, Homo sapiens is still locked in the same epic battle with viruses and other pathogens that we’ve been fighting since the beginning of our history. When cities first arose, diseases laid them low, a process repeated over and over for millennia. When Europeans colonized the Americas, smallpox followed. When soldiers fought in the first global war, influenza hitched a ride, and found new opportunities in the unprecedented scale of the conflict. Down through the centuries, diseases have always excelled at exploiting flux.