You are only allowed to ask one question at a time so I edited down accordingly. Clostridium difficile is a gram positive bacteria that can reside in the intestines of humans. Under normal circumstances, it is vastly outnumbered by the natural "gut flora", or the diverse array of normal, healthy bacteria that reside in the human gut to help break down food molecules for digestion and absorption. Clostridium difficile exists inside hard spores that help protect it. After taking a broad spectrum antibiotic that temporarily eliminates much of the gut flora, the C. difficile spores are unharmed and can now populate the intestines without competition from other bacteria. C. difficile can cause serious irritable bowels and abdominal pain if left untreated. It requires a more harsh antibiotic treatment.