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Pain has a profound effect on our bodies - when we are experiencing it, millions of nerve cells deep within our brains are firing, telling us 'it hurts' - and for centuries the challenge has been to find something that will lessen or even switch off these sensations to bring us relief. Dr Michael Mosley discovers just what pain is, why we want to control it and how we ultimately did it when the discovery of morphine, the world's first pharmaceutical, at the beginning of the 19th century led to a 200-year journey of scientific breakthrough, discovery and self-experimentation.

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.

Dr Michael Mosley ends the series with a look at poisons, exploring the turning points when scientists went from finding antidotes to poisons to applying poisons as a cure - celebrating the eccentrics and mavericks whose breakthroughs were to pave the way for some of the most striking treatments of modern medicine. Of the medicines explored in this series, those that are derived from poisons are perhaps the most extraordinary. The story of turning poisons into medicines encompasses the planet's most deadly substances, in which we turned killers into cures.