Just months ago, the world stood in fear of an emerging new disease that threatened to kill millions. A new flu variant H1N1 had arrived. In the UK alone, 65,000 deaths were predicted. Yet to date, these dire warnings have not materialised. If this latest pandemic has taught anything, it is just how little is understood about the invisible world of viruses. But that has not stopped scientists trying. Horizon follows the leading researchers from across the world, who are attempting to unravel the many secrets of viruses to understand when and why they kill.