There isn't a single virus you'd want on your computer, but there are a few that you really don't want. One of the worst is CryptoWall, the successor to the infamous CryptoLocker ransomware.
Like the name implies, ransomware encrypts your files and holds them hostage unless you pay up. In the case of CryptoWall, the price starts at $500 and you only have a limited time to pay up.
If you don't pay, your files are locked for good - unless you have a backup, which you should. Even some police departments have paid off CryptoLocker and CryptoWall hackers to get their files back. That's how serious it is.
As a bit of good news, though, the original CryptoLocker is all but dead. The FBI took down its control servers and security experts manage to crack its encryption so you can get your files back.
Unfortunately, CryptoWall isn't going to be as easy - if you can call it easy - to defeat. It's built to avoid the weaknesses that led to CryptoLocker's downfall.
Even worse, it's being spread in a way that's hard to avoid. You might have even come in contact with it in the last month.