The average human digestive tract is home to as many as 1,000 species of microorganisms. Most of them are harmless -- or even helpful -- under normal circumstances. But when something upsets the balance of these organisms in your gut, otherwise harmless bacteria can grow out of control and make you sick. One of the worst offenders is a bacterium called Clostridium difficile(C. difficile, or C. diff). As the bacteria overgrow they release toxins that attack the lining of the intestines, causing a condition called Clostridium difficilecolitis.