Viruses have shaped our health and our history, and, despite all the tools of modern medicine, they continue to kill millions of people every year. Influenza, smallpox, and Ebola are among the three most lethal viruses ever to have plagued mankind. Each has taken a devastatingly large toll on the human population. Smallpox killed more people than all the wars in human history, and we are just one test tube away from biomedical warfare. The flu spreads like wildfire across the globe every year, killing the young and the old alike, and Ebola shocks and terrifies the world each time it emerges. The ferocity of these viruses is anything but an event of the past; according to recent reports from the U.S. Centers for Disease Control and Prevention (CDC), the 2017-2018 flu season is one of the worst in years. Smallpox, eradicated in the wild, is a top bioterrorism threat. And the next Ebola outbreak always lurks just out of sight.

Three years in the making, Discovery's three-part series Invisible Killers takes viewers around the world to understand how viruses have shaped our health and history, the biological and social impact they have on our global society, and the incredible work being done to combat them. In the ongoing battle between humans and viruses, Invisible Killers asks: Are we winning? And, when the next pandemic comes, will we be ready?