Nearly 100 countries and tens of thousands of computer systems were hit by a cyber attack of unprecedented proportions utilizing a program believed to have come from the National Security Agency (NSA).

It is believed that over 75,000 computers were infected mainly in Britain, Spain, Russia, Ukraine and Taiwan with the virus now dubbed ‘Wanna Cry’. The software sent out emails which appeared to contain invoices, job offers, security warnings and other legitimate files.  When opened, the virus locked users out of their systems, demanding a ransom of $300-$600 be paid or data would be destroyed. It was unclear whether people who paid were allowed back into their systems.

In Britain, hospitals were locked out of their systems, forcing  emergency rooms to turn away patients. The virus froze approximately 1,000 computers in Russia’s Interior Ministry and police.

The virus takes advantage of a vulnerability in Windows that was identified by the NSA. The information was allegedly stolen from the NSA in April by a group called the Shadow Brokers, but may have been passed to other hackers as well. Microsoft released a patch in March to address this vulnerability, but networks that did not adopt it  remain vulnerable. On Saturday, Microsoft released an additional security update for all customers to further protect Windows platforms.