Every once in a while I come up with a horrifying idea that is at least somewhat plausible, while also being so awful I hope it's never created in reality.

Today's nightmare is a retrovirus containing a combination of genes that together cause human cells to create additional vacuoles, break down fats into ketones, and then use them to make explosive organic peroxides which are then stored in the newly-created vacuoles... slowly transforming the unwitting infected person into a living bomb.