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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!'

But what followed was far from civilised. It was very clear that all the data on an infected machine was now scrambled and only the hackers could unscramble it. For a price - and with an extra twist - after a few days the ransom money doubled, and if nothing was paid within a week, the hackers threatened to destroy all the data - forever.