Zonbi asu (2011)

Wracked with guilt over the suicide of her bullied sister, young karate student Megumi accompanies four older friends on a trip into the woods: smart girl Aya, her druggie boyfriend Take, full-figured model Maki, and nerdy Naoi. Things start to go badly when Maki finds a parasitical worm inside a fish - and wolfs it down, in the hope that it will keep her skinny. Her stomach later feels horrible and she relieves herself in an outhouse. The parasitic worm she ate had apparently laid eggs in her stomach and came out of in her diarrhea attack. Soon after, they are attacked by a crowd of poo-covered undead, who emerge from the outhouse toilet Maki used. The group seeks refuge in the home of the strange Dr. Tanaka, who conducts experiments on the zombies and the parasites, and who has another fate in mind for the five strangers. Can Megumi's karate help them escape, or will she have to rely on the liberating power of flatulence to defeat her demented captor?