We may fear terrorist attacks, but in truth humans have always had far more to fear from infections. In 1919, Spanish flu killed over 50 million people, more than died in both world wars combined. In 1950, an estimated 50 million people caught smallpox worldwide, of whom 10 million died. In 1980, before measles vaccine was widely used, an estimated 2.6 million children died of measles every year. Less than 100 years ago, losing a child to an infection like diphtheria or polio was a dreaded but almost inevitable sorrow faced by all parents, from the richest to the poorest. Today, these killer diseases are almost never seen in industrialised countries, thanks to the development of vaccines. Immunisation has given modern parents peace of mind their ancestors could not imagine. The history of vaccination is rich with trial, error, sabotage and success. It encompasses the tragedy of lives lost, the drama of competition and discovery, the culpability of botched testing, and the triumph of