It’s a virus, a retro-virus at that. Well, that’s what some researchers are speculating. And they do have some interesting evidence.
I can’t help thinking it’s not going to be this simple. But I was wrong once before (years ago, of course) 
, and even if this isn’t the cause, it may be a part of the story. Of course they haven’t developed a cure for the virus, yet. But at least it presents a target to aim at.
Here’s the story from NPR: