What 'progressive' means

At our last support group meeting, they gave May a book on "progressive memory loss" and asked her to bring it back this week so someone else could read it. She's been working on it every day, like a homework assignment. Yesterday she looked up from her reading and said, "So, do I have progressive memory loss?" I said yes. She looked concerned and I said, "You know when you leave your purse somewhere and we have to go back for it and I show a hint of displeasure and then you say, 'You know, it's only going to get worse?'" She recognized that scene. "Well," I said, "that's what you're talking about--it's only going to get worse because it's progressive." That seemed to satisfy her.