Based on a true story told by Dr Oliver Sacks, Awakenings focuses with exquisite detail on the experiences of a few extraordinary people affected by a disease known as encephalitis lethargica, or the “sleepy sickness”. Yet far from being a rarity, this disease affected a million people worldwide during and after the first world war. Then it vanished and has remained a mystery for the past century. The question that has never been answered is: what caused it?

The disease was first described by a neurologist in Vienna in 1917. It was noted that the initial symptoms were similar to those of flu, but that’s where the similarities ended. Over the next few weeks, some would be unable to sleep at all, while others would be so drowsy they could be woken for only a few minutes to eat.

About half died in this early phase, but those who survived were even more perplexing. After recovering, often returning to work, many started to notice stiffness, slowness in their movements and even that their eyes would get stuck in certain positions. Sadly, this slowly progressed. And many were left – like De Niro’s character – in a frozen state, unable to speak or move.