CryptoWall is classified as a Trojan horse, which is known for masking its viral payload through the guise of a seemingly non-threatening application or file. Its payload involves encrypting the files of infected computers in an effort to extract money for the decryption key.
CryptoWall and viruses similar to it are also known as "ransomware" in that the infection offers the end user a means with which to remove the threat and recover all their files in exchange for paying a ransom. After they pay, the user is allowed to download and run a file and/or application to cleanup the infection or, in this case, decrypt the encrypted files to return them back to a working state.
There are literally dozens of different ways a computer can become infected with spyware, viruses, and other malware. The following document contains is a list a number of these infection methods; listed in the order we believe to be most to least common.
No Antivirus or Spyware Scanner
If you're running a computer with Microsoft Windows, we highly recommended you have some form of antivirus and spyware protection. This software removes existing (if any) viruses and helps prevent future infections.
Downloading any infected software
When acquiring any software (programs, utilities, games, updates, demos, etc.) via the Internet, make sure you're downloading the from a reliable source. Be sure to run your downloads through your antivirus and spyware scanners upon completion. As we stated in a previous section, during the installation process, reading all prompts about what the program is putting on your computer.
Accepting without reading
By far one of the most common ways a computer becomes infected is when a user accepts what they see on the screen without reading the prompt before proceeding.