A woman I know contacted me for help with her computer and it turns out she was hit with a very nasty infection.
Something that seems to be gaining popularity with the criminals is variants of a thing called CryptoLocker (CryptoLocker 2.0, CryptoWall, etc). What it does is encrypt documents, images, and a large variety of other files it finds (not only on your C: drive, but also your mapped drives - dropbox, attached storage such as thumb drives, network attached storage, etc) and makes them completely inaccessible to you. It then demands a ransom payment to get your data back, which can run as much as $500 (payable in bitcoin).
This is distributed through a variety of ways, with some of the most common ones being those fake emails that say things like: 'we were unable to deliver your USPS parcel', 'we were unable to deliver your UPS parcel', 'there was an error processing your card payment', etc. However, it can also happen through malicious websites, malicious adverts, infected freeware and shareware, unsecured ports on your computer's internet connection, etc.
Dropbox can roll back the files you store on it, though it can be tedious unless you install a tool to automate the process, and if you're keeping shadow copies on your system, that can help as well. Better, though, is if you routinely make backups to either a service on the cloud or to something that doesn't stay connected to your system (such as DVDR discs).
If you search on the internet, you find that the number of people getting hit by these is huge, with systems being hit ranging from the household user to big corporations.
That woman's experience is a good reminder to all of us to be more diligent in making backups. And, of course, to be very careful about what you click on and what email attachments you open.