CryptoWall is considered to be the largest and most destructive ransomware threat on the internet.
From Trendmicro: Ransomware is a type of malware that prevents or limits users from accessing their system. This type of malware forces its victims to pay the ransom through certain online payment methods in order to grant access to their systems, or to get their data back. Some ransomware encrypts files (like CryptoWall).
The CryptoWall virus operates silently on your computer without your knowledge until it has finished encrypting all of your documents. At that point it locks your computer and presents a warning indicating that you must pay the ransom to retrieve a key and unencrypt your data. There is a limited time in which to retrieve the key and of course there are no guarantees that it will work.
Almost all users who encounter this virus are unable to retrieve their documents after the encryption has been completed.
In order to avoid the virus, NEVER open an email attachment that you are unsure of. This virus is mostly spread through fake invoices and delivery confirmations.
Also, make sure you are performing regular and complete backups of your important data, including pictures and documents, etc.
Finally, be sure that you have a good antivirus protection, that the subscription is current and that it has been properly updated.
If you suspect any virus related activity, immediately shut your computer off and bring it in for a check up.