John Clark, professor of computer and information security at the University of Sheffield, said the attack should act as a wake-up call for the health service and for businesses and individuals to defend themselves adequately against so-called "ransomware".
The initial attack, known as "WannaCry", paralyzed computers running Britain's hospital network, Germany's national railway and scores of other companies and government agencies around the world.
After cancelling operations for a number of days, Lancashire Teaching Hospitals NHS Foundation Trust, which runs Chorley and Preston hospitals, is now advising patients to attend appointments as normal from today.
The following screen pops up when people have been hacked by the "Wannacry" virus.
It was all hands on deck as cyber security experts in the National Health Service teamed up with the National Cyber Security Centre to patch outdated computer systems last week.
"Our staff and partners worked tirelessly around the clock to ensure there was as little interruption to patient care and experience as possible and this meant that wewere able to continue to offer our services to patients as near to normal as possiblethroughout". He said: "We had no computers and no access to any records".
In Russia, where a wide array of systems came under attack, officials said services had been restored or the virus contained.
"It is important to note that the vast majority of NHS organisations report that they are running contemporary IT systems, which are commissioned depending on local need", the NHS said in a statement.
She said Britain was working with worldwide partners in the global manhunt to find the creators of the cyber attack.
On Friday, Microsoft released patches to fix a vulnerability in older software.
Mikko Hypponen, chief research officer at the Helsinki-based cyber security company F-Secure, told AFP it was the biggest ransomware outbreak in history, saying that 130,000 systems in more than 100 countries had been affected.
Qihoo has previously said the attack had infected close to 30,000 organizations by Saturday evening.
Meanwhile, an executive at a cybersecurity firm that helped block Friday's attack said that new variations of the malicious worm are circulating - and that researchers expect one to develop that can not be stopped. Luckily, though, the spread of "WannaCry" has slowed down over the weekend.
However, the wave of attacks has slowed down significantly.
The National Computer Network Emergency Response Technical Team/Coordination Center of China (CNCERT/CC) had found 2.42 million IPs were suffering the attack and the number of infected IPs reached 35,000 as of 10:30 am on Sunday, Xinhua reported. Here, victims of the cyberattack can receive instructions on how to potentially unlock their computers without having to shell over any money to the attackers.
May added: "Cyber security is an issue that we need to address".
The attack grew over the weekend after emerging on Friday afternoon (12 May) from 45,000 victim systems to an estimated 200,000, crippling large organisations from the NHS in the United Kingdom to Renault factories in France, Telefónica in Spain as well as Russia's second largest mobile operator, MegaFon.
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