Infection can take over the entire human body, and if our immune systems aren't strong enough we will die - in fact, infectious disease has regularly wiped out millions of people across the planet. Dr Michael Mosley explores our earliest attempts to tackle infection and reveals the moment we began to harness the power of microbes to fight back. This is the story of how scientists, chemists and doctors helped us win the battle, from Louis Pasteur to Howard Florey, and how a small team of dedicated men and women wiped out one of mankind's deadliest diseases - smallpox.