No one knows what causes it. It's not passed in the air. It's not sexually transmitted. It's not a virus or a bacteria, or if it is, it's nothing scientists have been able to find. At first everyone blamed the fashion industry, then the millennials, and, finally, the water. But the water's been tested, the millennials aren't the only ones going incorporeal, and it doesn't do the fashion industry any good to have women fading away. You can't put clothes on air. Not that they haven't tried.
"An unread book holds wonderful unknown promise, but also threatens disappointment. A re-read is a known quantity."
- Jo Walton, What Makes This Book So Great?