The BBC News informs us that "faecal transplant" is the "clue to treating the gut bug" Clostridium difficile (C. difficile).
C. difficile are bacteria that usually survive in the digestive system (gut) and do not cause any problems in healthy people. Sometimes, when people are given antibiotics to treat other illnesses, the balance of 'friendly' bacteria in the gut is disturbed. When this happens, C. difficile can multiply rapidly and produce toxins (poisons), which can cause severe diarrhoea. As C. difficile infections are usually follow antibiotic treatment, most cases happen in hospitals or care homes.