Back up! And disconnect it. Here's why.

There is a new (past year or so) strain of viruses that are considered "Ransomware" which encrypt you data and then demand payment for decryption. Search for "CryptoWall 2" for details. Government entities have paid the ransom after infection because they couldn't crack it. Basically the virus looks for certain types of files and encrypts them, then deletes the originals. "Restore points" are also deleted. It's serious stuff. It will look for files on any drive that is visible to your computer; anything that is mapped to a drive letter or equivalent. Ransoms are typically in the $500 range but increase over time. Again, read about it rather than taking my word for it.

I get the feeling it only targets Windows machines but I wouldn't expect that to remain true for long. Infection seems to come via email attachments (the old tried and true way of spreading viruses) and through Flash and Java.

If your computer gets infected, the only files that are safe are those that are not on an accessible drive. That means files that are in cloud storage would be safe at first, but your backup routine could overwrite those files with encrypted ones. You need to think this through for your particular case. Backing up to an external drive and then disconnecting it sounds like overkill but it might be the way to go.

There are some anti-virus products that will guard against it.