Malware is a general term that refers to software that's harmful to your computer, said John Villasenor, a professor at the University of California, Los Angeles.
"There will be lessons to learn from what appears to be the biggest criminal cyber-attack in history", Rudd said in response to a letter from opposition Labour Party spokesperson Jonathan Ashworth.
Friday's attack was the latest in the growing menace of ransomware in which hackers deliver files to computers that automatically encrypt their data, making it unusable until a ransom is paid. Questions have been raised over Trident's vulnerability to potential cyber attacks, as the system is believed to use ageing Window's XP software - the same operating system targeted by the global ransomware attack. "So they no longer get the security updates they should be".
Doctors surgeries in Tayside affected by the UK-wide NHS cyber-attack are now "operating as normal". Two security firms, Kaspersky Lab and Avast, said Russian Federation was hit hardest.
Anti-virus experts Symantec said the majority of organisations affected were in Europe.
There are several factors in play.
Europol, the European Union's policing agency, said the attack remains an "escalating threat" whose numbers "are still going up" after a brief slowdown on Friday.
Jan Op Gen Oorth, spokesman for the Netherlands-based Europol, said the number of individuals who have fallen victim to the cyberextortion attack could be much higher.
A doctor who said hospitals could be vulnerable to ransomware one day before a virus struck the NHS has warned this could "just be the beginning".
The anonymous specialist, known only as MalwareTech, prevented more than 100,000 computers across the globe from being infected.More news: Trump says wants to move quickly to nominate new Federal Bureau of Investigation director
Once it has infected a computer, the malware exploits a vulnerability in unpatched Windows computers to spread to networked computer systems. On top of that, copycat versions of the malicious software have already started to spread. The attacks used to only be able to target one machine at a time.
He warned governments against stockpiling code that could be used in this way left it fall into the wrong hands and said instead they should point out the vulnerabilities to manufacturers.
A ULHT statement said: "Due to the ongoing issues relating to the cyber attack on the NHS on Friday, we will be cancelling all routine activity in our hospitals on Monday 15 May".
Germany's rail operator Deutsche Bahn said its station display panels were affected.
Hitachi spokeswoman Yuko Tainiuchi said emails were slow or not getting delivered, and files could not be opened. The Central Bank of Russian Federation says it was able to detect the attack and stop it without any information being compromised.
The Federal Government says only three Australian companies have been confirmed as being hit so far, but says more could find they have been compromised when staff turn on their computers this morning. The company said the virus has been localized and "technical work is underway to destroy it and update the antivirus protection".
Tom Bossert, a homeland security adviser to President Donald Trump, said "criminals" were responsible, not the USA government.
A computer screen shows an error message after Britain's National Health Service was hacked. "At this stage, we do not have any evidence that patient data has been accessed".
"If you have a hospital appointment you should still attend unless you are contacted and told not to".