A few weeks ago, the National Health Service was hit by a widespread and devastating cyber attack - Horizon tells the inside story of one of the most challenging days in the history of the NHS. On the morning of 12 May the attack started. Appointment systems, pathology labs, x-rays and even CT scanners were infected - putting not just data but patients lives at risk, and on every screen a simple - some may even say polite - message appeared. 'Ooops, your files have been encrypted!'