Malicious software called 'ransomware' has forced British hospitals to turn away patients and affected Spanish companies such as Telefonica as part of a global outbreak that has affected tens of thousands of computers.
What is WannaCry - also known as WanaCrypt0r 2.0, WannaCry and WCry?
WannaCry is a form of "ransomware" that locks up the files on your computer and encrypts them in a way that you cannot access them anymore.
Ransomware is a program that gets into your computer, either by clicking on the wrong thing or downloading the wrong thing, and then it holds something you need to ransom.
In the case of WannaCry, the program encrypts your files and demands payment in bitcoin in order to regain access.
Security experts warn there is no guarantee that access will be granted after payment. Some ransomware that encrypts files ups the stakes after a few days, demanding more money and threatening to delete files altogether.
There are different variants of what happens: Other forms of ransomware execute programs that can lock your computer entirely, only showing a message to make payment in order to log in again. There are some that create pop-ups that are difficult or impossible to close, rendering the machine difficult or impossible to use.
British based cyber researcher Chris Doman of AlienVault said the ransomware "looks to be targeting a wide range of countries", with initial evidence of infections in at least two dozen nations according to experts from three security firms.
The broad based ransomware attack has appeared in at least eight Asian nations, a dozen countries in Europe, Turkey and the United Arab Emirates and Argentina and appears to be sweeping around the globe, researchers said.