In 1918, shortly before the end of the First World War, a virus erupted around the world, unleashing the deadliest outbreak of disease in human history. Scientists now put the death toll from the 1918 flu pandemic at 50 million people worldwide. But where did it come from? How did it spread so far and so fast? And why did so many people die? It's a mystery that has yet to be resolved. Dr Jonathan Miller hears from scientists, historians and survivors, and asks whether we are at risk of another flu pandemic, just as deadly as that of 1918. Producer John Murphy How vulnerable are we now?: page 115