Also titled "Blanket." Chapter one of Ma no Kakera.
A woman (unnamed in the original but named Madoka in the Viz translation) has eloped with her boyfriend, Tomio. Shortly after they move into their new home, he begins hiding under a blanket on his futon and won't come out. He tells her that there are demons everywhere and she should hide from them too. Tomio won't see a psychiatrist and refuses to leave his futon, even to go to the bathroom. His girlfriend becomes exhausted from the stress of working all day and then taking care of him at home.
One night, the woman wakes to see that the room is full of hideous demons. Tomio points out a demon in the form of a naked woman with a tail. He says that one night when his girlfriend was away, he met the demon on the street and was seduced by her in the belief that she was an ordinary human. Ever since then, he has been able to see the demons.
Tomio's girlfriend runs screaming out of the apartment and doesn't come back for a month. When she returns, he and the futon are covered in a strange, glowing bacteria. Tomio narrowly escapes death, and tests reveal that the bacteria cause hallucinations.