Chronic stress reaction versus acute on chronic stress reaction in a runner

I just had a consultation with a runner who had foot pain that she thought might be a stress fracture that had been coming and going for about a month. 

She had been increasing her training volume and doing strength exercises that are supposed to build her running fitness. 

The pain seemed to be volume related. The more training she did, the more she would notice the discomfort in her foot.

But when we x-rays of her foot, the radiologist suggested this could be an "acute on chronic" stress reaction. 

What does that mean? 

What is the difference between a chronic stress reaction, or an acute on chronic stress reaction in a runner? 

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