Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-74.196.206.251-20130817054539/@comment-13899768-20130817064319

Watch Episode 10 - "The Overlooked" and Episode 11 - "Alpha Pact"

They explain exactly what you're asking.

In short, if you don't want to watch, Cora was poisoned with Mistletoe.