 The exclusive story of scientists around the world who are in race against time to find the secret of flu pandemics that wipe out millions. The worst one in recorded history was in 1918 and experts fear another one could be just around the corner. The 1918 Spanish flu pandemic killed an estimated 40 million people - more people than died in the First World War - and was the biggest infection in history. Many of its victims were young, fit people. The virus killed in an unusual way, by flooding the lungs with fluid and causing haemorrhaging. It died out as mysteriously as it began. But despite the pandemic s impact, scientists still do not know what caused it. One expert called it "an 80-year-old murder mystery" and this TV special investigates how it is about to be solved.