Some of the simplest sources for a horror film are different or twisted perspectives on common occurrences. Gremlins is ultimately a film about bringing home a pet as a gift for a loved one. The Conjuring is about the fear of the monster hiding in the closet. The Shining is about anxiety from staying in a hotel far away from home. Little things we take for granted can be terrifying with the right perspective.
The Cleanse is a horror comedy about cleanses.