The FBI and Europol seized servers used to distribute the CryptoLocker malware. The malware, like its descendants Cryptowall, encrypts the contents of victims PCs and demands a payment, usually in Bitcoin, in order to get the decryption key. With ransom payments ranging from less than $100 to as much as $300 or more, the criminals behind these threats are building multimillion dollar businesses on the fear and desperation of their victims. The most common method is perhaps the most common infection is through spam messages.
Source: https://threatpost.com/ransomware-looming-as-major-long-term-threat/111265/