Irritable bowel syndrome (IBS) or spastic colon is a symptom-based diagnosis. It is characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits. Diarrhea or constipation may predominate, or they may alternate (classified as IBS-D, IBS-C, or IBS-A, respectively). As a functional gastrointestinal disorder (FGID), IBS has no known organic cause. Onset of IBS is more likely to occur after an infection (postinfectious IBS-PI), or a stressful life event, but varies little with age. The most common theory is that IBS is a disorder of the interaction between the brain and the gastrointestinal tract. For at least some individuals, abnormalities in the gut flora occur, and it has been theorised that these abnormalities result in inflammation and altered bowel function. A diagnosis of IBS may be made on the basis of symptoms, in the absence of worrisome features such as age of onset greater than 50 years, weight loss, bloody stool, signs of infection or colitis, or family history of inflammatory bowel disease. Routine testing yield no abnormalities, although the bowels may be more sensitive to certain stimuli, such as balloon insufflation testing. Several conditions may present similarly, including coeliac disease, fructose malabsorption, mild infections, parasitic infections like giardiasis, inflammatory bowel disease, bile acid malabsorption, functional chronic constipation, small intestinal bacterial overgrowth, and chronic functional abdominal pain. Although no cure for IBS is known, treatments to relieve symptoms exist. This including dietary adjustments, medication, and psychological interventions. Patient education and good doctor-patient relationships are also important. Dietary measures that have been found to be effective include increasing soluble fiber intake. IBS has no direct effect on life expectancy. It is, however, a source of chronic pain, fatigue, and other symptoms, and contributes to work absenteeism. It is common and its effects on quality of life make it a disease with a high social cost. Psychiatric disorders such as anxiety and major depression are common in IBS.