In May 2017, a notorious ransomware attack known as WannaCry spread rapidly across the globe. Targeting computers running Windows OS, WannaCry encrypted files to make them impossible to access—then demanded a ransom in bitcoins in order to decrypt them. Hundreds of thousands of computers were affected, impacting government offices, railway networks and businesses worldwide. In the UK, hospitals were knocked offline and the National Health Service had to turn away non-critical emergency patients.