A cyber-attack on the NHS has forced hospital wards to close, patients to be turned away and operations to be cancelled as at least 30 health service organisations in England and Scotland were infiltrated by the malicious software.

Read more: Global ransomeware attack: Keep your business safe

What exactly is ransomware, how is it spread and what happens next?   

The ransomware took hold on Friday afternoon, locking machines and demanding money to release the data.

It was reported up to 99 other countries had fallen victim to the attack – including the US, Russia, Spain and India – which had targeted a multitude of hospitals, companies and government agencies.

The malicious software appeared to exploit a vulnerability in Microsoft Windows that was supposedly identified by the US National Security Agency for its own intelligence-gathering purposes and was later leaked to the internet.

Read more: From virus protection to safe passwords - how BT can keep you safe online