Clostridium difficile, spindle, and Latin difficile, difficult), also known as "CDF/cdf", or "C. diff", is a species of Gram-positive bacteria of the genus Clostridium. Clostridia are anaerobic, spore-forming rods (bacillus). C. difficile is the most serious cause of antibiotic-associated diarrhea (AAD) and can lead to pseudomembranous colitis, a severe infection of the colon, often resulting from eradication of the normal gut flora by antibiotics. The C. difficile bacteria, which naturally reside in the body, become overpopulated: The overpopulation is harmful because the bacterium releases toxins that can cause bloating, constipation, and diarrhea with abdominal pain, which may become severe. Latent symptoms often mimic some flu-like symptoms. Discontinuation of causative antibiotic treatment is often curative. In more serious cases, oral administration of metronidazole or vancomycin is the treatment of choice. Relapses of C. difficile AAD have been reported in up to 20% of cases.