Getting started with the virus that's already affected more than 200,000 victims worldwide. But it's not a biological outbreak that makes people sick, it's a cyber attack. It started on Friday.
Its spread was temporarily slowed down over the weekend by a cyber security researcher, but the criminals behind the attack found a way to get the virus spreading again. And experts were concerned that Monday morning, when people got back to work, and started up their computers, the number of infected machines would jump.
The problem is caused by a type of malware, software that can damage computer systems. What it's doing is locking up computers that used Microsoft Windows and holding them ransom. If owners pay the ransom, between $300 and $600, they'll get access to their machines back.
The thing has spread far and wide. It's been detected at least 150 countries. International companies like FedEx and Nissan had been targeted. Russia's Central Bank was reportedly attacked but wasn't hurt by the virus. Other governments and businesses were taking steps to either contained or prevent the virus altogether.
And there is something people can do to protect themselves.
SAMUEL BURKE, CNN TECHNOLOGY CORRESPONDENT: Here's what you need to know about the ransomware that's been dubbed "WannaCry".
Security experts say this is one of the worst and most widespread pieces of malware they've ever seen, especially because it's even caused some hospitals in the U.K. to have to cancel outpatient appointments.
So, what exactly happened to infected computers and how big is this?
The ransomware actually locks up all the files in your computer and demands $300 in bitcoin in order to regain control. People are seeing this message all around the world. Researchers say this is spreading through a Windows' weakness known as eternal blue, which Microsoft released a patch for last month.
This ransomware is actually just going through the Internet looking through vulnerable computers, according to cyber security firm Malware Bites. That means you don't even have to click a phishing email to get infected.
How can you protect yourself? Well, you know those seemingly annoying security updates from Microsoft Windows, if you've installed the latest one, you're safe. If you haven't, do it right away. For now, that's about it.
Who's responsible? Researchers aren't pointing their fingers yet, but the most amount of attacks so far have been in Taiwan, Ukraine, and Russia, according to Avast.