The first known ransomware attack was called the AIDS Trojan and was unleashed via floppy disks in 1989, but it wasn't until the global panic caused by WannaCry in 2017 that the malware entered the public consciousness.
The rise of bitcoin as an almost untraceable payment and the growing set of evidence that digital extortion could be profitable made ransomware a common cyber threat by 2016, according to Kaspersky Lab. It continued to wreak havoc in 2017 and its surge shows little sign of respite.
Ransomware is difficult to stop even for Windows computers running antivirus, although that situation is improving. The only reliable defence is a backup, but even that can come under attack if it can be reached from the infected PC.
As the threat is unlikely to subside in the foreseeable future, we look at the most deadly strains of ransomware to have emerged so far.