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? The series features interviews with officials from the CDC, World Health Organization, National Institutes of Health (NIH), National Institute of Allergy and Infectious Diseases (NIAID), and insights from experts at the United Nations along with homeland security officers and biosecurity experts. INFLUENZA delves into the astonishing and little-understood history and impact of a constantly evolving virus, which is often tragically underestimated as merely a cousin to the common cold. Each year, six months in advance of the anticipated flu season, world health officials gather to speculate what strain of virus will be most prevalent and determine which vaccine will be most successful in treating those affected. “We’ve got to be able to look in our crystal ball and say, ‘Okay, six months from now which ones of these thousands and thousands of strains are likely to be circulating then? What’s our best guess?’” explains Dr. David Morens, Senior Scientific Advisor, NIAID/NIH. “And then we have to make that vaccine.” 100 years later, the episode revisits the catastrophic 1918 flu pandemic, in which there were no vaccines available, and which killed an estimated 50 to 100 million people around the world, 700,000 in the U.S. alone. Fast forward to current times, where flu vaccines are available at low-cost to millions across the country but are mistakenly believed by many to cause the illness – a belief health officials urge is simply not the case. Anthony Fauci, Director, National Institute of Allergy and Infectious Diseases, explains, “the extraordinary potential influenza has to devastate certain segments of the population. It can be explosive.” SMALLPOX tells the story of perhaps the greatest accomplishment of medical science, the complete eradication of the worst disease known to humankind, a scourge responsible for an estimated 500M deaths in the 20th century alone. Journalist Michael Specter explains, “It's probably killed more people than all the wars in all of human history combined.” Now, with the disease eradicated for nearly 40 years, this once feared and deadly virus could return – this time as one of the most significant bioterror threats facing humankind. “This is what a weapon of mass destruction in the 21st century looks like,” states COL. Randall Larsen, USAF (RET), and Director, Institute for Homeland Security states, “and if this were dry-powder variola virus, you carry half a pound in a briefcase and release it on the subway system and it will change the course of history.” EBOLA examines one of the world’s deadliest viruses - spread by physical contact with infected body fluids – as it turns human compassion in to human vulnerability attacking the individuals and health professionals trying to care for the sick. The episode looks back at the perfect storm of the 2014 West African outbreak - where lack of public health infrastructure, cultural practices, and a slow global response produced the deadliest outbreak on record. Seema Yasmin, CDC epidemiologist states, “The outbreak in West Africa showed an outbreak anywhere is a threat everywhere so you have to be ready at all times.” The hour also reveals the dramatic race to develop a and test it before the outbreak subsided and the breakthrough success that they had.