There are a number of ways ransomware can take to access a computer. One of the most common delivery systems is phishing spam — attachments that come to the victim in an email, masquerading as a file they should trust. Once they’re downloaded and opened, they can take over the victim’s computer, especially if they have built-in social engineering tools that trick users into allowing administrative access. Some other, more aggressive forms of ransomware, like NotPetya, exploit security holes to infect computers without needing to trick users.

There are several things the malware might do once it’s taken over the victim’s computer, but by far the most common action is to encrypt some or all of the user’s files. If you want the technical details, the Infosec Institute has a great in-depth look at how several flavors of ransomware encrypt files. But the most important thing to know is that at the end of the process, the files cannot be decrypted without a mathematical key known only by the attacker. The user is presented with a message explaining that their files are now are now inaccessible and will only be decrypted if the victim sends an untraceable Bitcoin payment to the attacker.

In some forms of malware, the attacker might claim to be a law enforcement agency shutting down the victim’s computer due to the presence of pornography or pirated software on it, and demanding the payment of a “fine,” perhaps to make victims less likely to report the attack to authorities. But most attacks don’t bother with this pretense. There is also a variation, called leakware or doxware, in which the attacker threatens to publicize sensitive data on the victim’s hard drive unless a ransom is paid. But because finding and extracting such information is a very tricky proposition for attackers, encryption ransomware is by far the most common type.

There are several different ways attackers choose the organizations they target with ransomware. Sometimes it’s a matter of opportunity: for instance, attackers might target universities because they tend to have smaller security teams and a disparate user base that does a lot of file sharing, making it easier to penetrate their defenses.

On the other hand, some organizations are tempting targets because they seem more likely to pay a ransom quickly. For instance, government agencies or medical facilities often need immediate access to their files. Law firms and other organizations with sensitive data may be willing to pay to keep news of a compromise quiet — and these organizations may be uniquely sensitive to leakware attacks.

Healthcare organizations ranked no. 3 on BitSight Insight’s top list of ransomware targets. “Hospitals, in particular, may pay the ransom because their patient data is critical in life-or-death situations,” the report noted. One such example was the Hollywood Presbyterian Medical Center, which paid a $17,000 ransom in 2016 to hackers who had locked some of the hospital’s critical data.