George says: One word: Glue. Wall paper is adhered to the wall with glue which is a favorite food source of mold. The wall paper itself is usually either paper (another organic food source for mold) or vinyl. Mold spores live around us naturally. So when mold spores find food sources then moisture is added, they settle in and spawn. Moisture can come from a water leak or humidity build-up such as a poorly ventilated bathroom or an air conditioner that fails. A word of caution to those who remove wall paper: take the time to scrape all glue off the wall. Don’t cheap out and try to paint over the glue residue. All it takes is some humidity and the glue will grow mold. It takes some elbow grease and often retexturing the wall to get a good finish, but in the end you’ll have a beautiful wall and peace of mind.