Do you use Transmission?
If yes, I suggest you check your version and update it soon. There are reports that a specific version has been tampered with.

Over the weekend, the first instance of Mac ransomware was found in a malicious update to the Transmission BitTorrent client. Version 2.90 of Transmission downloaded from the Transmission website was infected with "KeRanger" ransomware.

"Ransomware" is a class of malware that encrypts a user's hard drive and files, demanding money to decrypt it. In this case, KeRanger would have required Mac users to shell out a bitcoin for decryption, equivalent to approximately $400.
Read on here: macrumors.com

Also, more information about this ransomcrap (sorry, ware), you can find here:
http://researchcenter.paloaltonetworks.com/2016/03/new-os-x-ransomware-keranger-infected-transmission-bittorrent-client-installer/