What is the 'ransomware' WannaCry worm?

Malicious software called “ransomware” has forced British hospitals to turn away patients and affected Spanish companies as part of a global outbreak that has affected tens of thousands of computers.
 
How does it work?
 
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.
 
How does it spread?
 
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.
 
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