The New York Times has recently written an article about the CryptoWall malware.
My mother received the ransomnote on the Tuesday before Thanksgiving. It popped up on her computer screen soon after she’d discovered that all of her files had been locked. “Your files are encrypted,” it announced.
This type of malware seems to be a very good reason to have a backup system in place. However, a local backup drive that is attached at the time of infection could also easily become encrypted too so a cloud backup service like Backblaze or CrashPlan could be useful.