The Black Death in 1348 was the worst disaster ever to hit London. It was a brand new disease, which swept through the city, killing old and young, rich and poor, within days of their first symptoms. But exactly how many died, and why they died, has long been a mystery. Received wisdom has long suggested that the culprit was bubonic plague, spread by the fleas of infected rats. Now 25 skeletons uncovered during the construction of Crossrail, Europe's largest engineering project, could settle the argument. This Secret History documentary follows experts from a range of disciplines as they pick through the evidence and reveal why the plague killed on such a scale, and that it's still a threat today. In 2013, a team led by archaeologist Jay Carver dug a shaft on the edge of Charterhouse Square in central London and uncovered a large number of skeletons, neatly buried in layers. They had found a corner of a long-lost emergency burial ground, created by Edward III's men in 1348, when the Black Death reached British shores. DNA analysis proved that the skeletons died of bubonic plague; painstaking analysis of contemporary wills showed that more than 60% of Londoners died in that single apocalyptic year: the equivalent of nearly five million Londoners today. But there's nothing about bubonic plague that explains such a high death count. Bubonic plague is still around today, and kills in the dozens, rather than in the hundreds of thousands. Twenty-first-century pandemic experts from Porton Down reveal the real killer, as well as why London in 1348 proved the perfect soil for the disease to kill in such large numbers, and what this 650-year-old cataclysm can teach us. Every disease has its time and place; what is coming for us next? Secret History showcases the best in historical journalism.©Channel Four Publicity.