The beginning, of course, is that we have multiple demons running around. They pass between us every time we have sex with each other, and when you receive one of the demons, you have a vision of your own death. It's...paranoia, I guess. A way that people seem to feel they're going to die until they pass the demon on. From what I hear the chain starts early in the week, but the host themselves don't even know it.

It moves fast. Wei Wuxian seems to think that people who host it at least once still have it lingering, but obviously I haven't said anything without proof.