Difference between a mild stress fracture and a serious stress reaction in a runner?

I had a call with a runner who asked me an interesting question about the difference between a mild stress fracture and a serious stress reaction in a runner. She wanted to know if the difference was based on the amount of tenderness or pain that she had. 

Although it stands to reason that the more tenderness, the more pain you have, the worse it must be. However, that is not really what differentiates a mild stress fracture from a serious stress reaction. 

What's the difference between a mild stress fracture and a serious stress reaction in a runner? 

Well, that's what we're talking about today on the Doc On The Run Podcast.