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The Hollywood Presbyterian Medical Centre has been hit by "ransomware" - a kind of malware that encrypts the victims' data, making it useless unless they pay a fine.
Its computer systems have been useless for more than a week, NBC Los Angeles reports. Documents and medical records have to be written on paper, tests results have to be picked up in person, and some patients are being sent to other hospitals due to the hack.
It was reportedly a "random" - rather than targeted attack. According to CSO Online, the ransom is $3.6 million in bitcoin, a digital currency often favoured by criminals and hackers due to its anonymity.
The hospital's CEO, Allen Stefanek, told NBC Los Angeles that on February 5, staff noticed "significant IT issues and declared an internal emergency."
With ransomware, the victims' data is sometimes rendered permanently inaccessible if the ransom isn't paid before a certain deadline - though it's not clear if this is the case here.
The FBI is currently investigating.