I've heard of leaky gut but didn't really know anything about it. This was a good intro for me.
http://www.nytimes.com/2013/02/03/magazine/the-boy-with-a-thorn-in-his-joints.html
http://www.nytimes.com/2013/02/03/magazine/the-boy-with-a-thorn-in-his-joints.html