What is the Rakghoul plague?
Take a look here:
http://www.swtorstrategies.com/2012/04/swtor-world-events-are-happening-rakghoul-plague.html
It's a live event (if you're familiar with WoW it's like the Undead Invasion before WotLK) that has been deployed on the servers with the new patch. The link I provided should fill you in with all the informations you need, at least to start, considering it's unfolding right now.