Ransomware is a type of malicious software that is used to infect computer systems, which are then held for ransom. The Justice Department also helped seize computers related to a custom ransomware known as NetWalker. In recent months NetWalker was used to extort victims from Lorien Health Systems, a Maryland assisted-living facility for seniors; Crozer-Keystone Health System, a chain of four hospitals in Pennsylvania, Delaware, and New Jersey; and the University of California, San Francisco. The charges indicate how much of an uphill battle the U.S. faces in combating hackers.