AP Explains: What is ransomware?

LOS ANGELES (AP) — Computers across the world were locked up and users' files held for ransom when dozens of countries were hit in a cyber-extortion attack that targeted hospitals, companies and government agencies.

In most cases, the software infects computers through links or attachments in malicious messages known as phishing emails.

"The age-old advice is to never click on a link in an email," said Jerome Segura, a senior malware intelligence researcher at Malwarebytes, a San Jose-based company that has released anti-ransomware software.

The idea is to try to trick the victim into running a malicious piece of code.

"Ransomware, like the name suggests, is when your files are held for ransom," said Peter Reiher, an adjunct professor at UCLA who specializes in computer science and cybersecurity.

Users should also look for malicious email messages that often masquerade as emails from companies or people you regularly interact with online.