Security experts are studying a computer software virus that has affected computers in at least 150 countries. The cyber-attack, first reported last Friday, hit companies, government agencies and hospitals across the world. Attackers used malware, which infects computers and takes control of user data. The infection encrypts the records and blocks use of computer files. The virus can spread quickly to other electronic devices linked to the infected computer. The cyber-attack was described by experts as ransomware. They chose this term because the computer is held hostage until the hackers receive a ransom payment. Officials said affected computers received a message demanding payment of $300 to unlock data. Victims were told the ransom would rise to $600 within three days. Security experts said paying the money did not guarantee a fix for infected computers.