Cause of a rogue experiment on a diaper it caused it to become sentient and spread its virus to other humans and diapers making the infected want to spread the diaper virus as a thick poofy diaper is molded around there waist signaling their infection and may evolve into special infected if their lucky.

(Start how you want maybe in the middle of the outbreak or even before the outbreak. Maybe your a scientist that is at the location of day 1? You choose.)