That's almost as good as my idea. I thought like a really light version of COVID with nothing but light symptoms like fever and sneezing, but it would change your DNA just the right way, so you develop a hidden FFI prion infection, leading to you eventually not being able to sleep until you die.

By the time anyone realizes it is even a thing, nearly everyone would be infected, just a few prions would be 100% fatal, and there would be nowhere on Earth to hide.

Everyone dies from lack of sleep induced dementia.