Last May, scientists met in Geneva, Switzerland, to update the World Health Organization’s plans for pandemic preparedness. It looks like a crisis could arrive sooner rather than later. Thanks to climate change and drug resistance, a handful of deadly organisms are spreading across the globe; some are poised to make a comeback in the U.S. after decades of absence.

Ninety years ago the Spanish flu swept across the globe, killing between 50 and 100 million people in only a few months. Since then, the specter of another flu pandemic dealing death and woe around the world has periodically terrified the medical and popular communities. But scientists searching for ways to prevent a similar outbreak in the form of the H5N1 bird flu have found a cure for the deadliest flu in the most unlikely place: nonagenarian immune systems.

What makes a disease deadly in the twenty-first century? Medicine has never been more advanced; our understanding of spread and infection, never more sophisticated. And yet, we may be poised for the largest and most devastating pandemic the human race has ever encountered.