The Black Death wiped out one third of the population of Europe during its dreadful sweep across the continent. Spread by fleas, rats and human breath, victims experienced an agonisingly painful death as rank-smelling wounds tore apart their blackening skin. But there were survivors: people seemingly immune to the deadly bacteria. Geneticist Steven O'Brien visits the Derbyshire village of Eyam where he meets the descendants of some of those plague survivors and discovers that they still carry a vital, protective gene. Meanwhile, AIDS researchers in the US make a similarly remarkable discovery: some people are indisputably resistant to the HIV virus. The reason? They posses the same mutant gene found in descendants of the Eyam plague survivors. Could this gene hold the key to a cure for AIDS?