Just to chime in with this one. I just got infected by a real bad virus.. in the last 25 minutes or so. First proper one on my mac. Firstly I noticed Logic X had no samples then Safari kept quitting.. opened up firefox, after a short while that started quitting. Went to download Bit Defender from the App Store and the store app wouldn't open!
I've fixed it and I am currently running Bit Defender to double check the virus has gone, here's how I got my Macbook Pro working again...
I booted down, and booted up holding down CMD and r, then selected Disk Utility - First Aid - Verify Disk... Verify Disk never usually comes up with anything bad but this time it did. So it looks like the virus successfully corrupted something. Then select Repair Disk. After it's done it should say 'problem fixed'. I then went out of that and re started as normal.
Opened up everything that didn't work before and it all works.. samples back in Logic X, Safari worked etc etc.. :-) This worked for me so give it a go.
Still doing a scan so can't tell you if it's still lurking around somewhere. All I know is my Mac works right now.
Hope this helps someone. 🙂