I can't read japanese but what I understand a zombie apocalypse happened and all the Zombies that killed or ate people died off eventually, the only zombie left that survived are the prostitute ones that died when they were having sex and now they unconsciously and subconsciously have sex with random stranger's, some business man heard the rumors about these zombie girls and looked for them, and found some shady man who hearded them like sheep provided a service to this business man and boom the Zombie prostitution was born and it healed many people who were scarred by the apocalypse.