Shortly before the end of the First World War, in 1918, a virus erupted around the world, unleashing the deadliest outbreak of disease in human history.
Scientists now put the deathtoll from the 1918 flu pandemic at fifty million people worldwide.
But where did it come from? How did it spread so far and so fast? 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 in 1918.