Researchers with security software maker Avast said they had observed 57,000 infections in 99 countries, with Russia, Ukraine and Taiwan the top targets.
"Hackers will realise that hospitals can be hacked relatively easily, and of course that hospitals have very sensitive data and we need that to manage our patients and it's time-critical data".
In a post on its website, Britain's National Cyber Security Center said that by registering a domain name that unexpectedly stopped the spread of the malware, the anonymous Britain-based cyber specialist, known as Malware Tech, had prevented further infections and "already resulted in preventing over 100,000 potential infections".
He also warned hackers could upgrade the virus to remove the "kill switch" that helped to stop it. Researchers with the security software maker Avast said they had observed more than 126,000 ransomware infections, with 60 % of infected computers located in Russian Federation, followed by Ukraine and Taiwan.
Sir Michael ruled out concerns over the threat of viruses on Trident operating systems, after the global attack which hit 48 NHS trusts in England and 13 Scottish health boards.
Authorities in both countries said the attack was conducted using "ransomware" - malicious software that infects machines, locks them up by encrypting data and demands a ransom to restore access.
The apparent chink in the NHS's defences led to criticism of the Tories after the Government decided not to extend a £5.5m support deal with Microsoft for Windows XP in 2015.
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UK Home Secretary Amber Rudd said on Saturday that the problem was largely resolved but that "there's always more" that could be done to protect against computer viruses.
"This is a virus that attacked Windows platforms".
"I don't think it's to do with that preparedness".
However Ms Rudd was accused of "wild complacency" over her response and many questioned why Health Secretary Jeremy Hunt had remained quiet during the crisis.
He said: "We need to get to the bottom of why the Government thought cyber-attacks were not a risk, when a combination of warnings and plain common sense should have told ministers that there is a growing and risky threat to our cyber security".
"The Government likes to look tough, but this is an example of where it has left Britain defenceless".
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