How do you get irritable bowel syndrome?

Irritable bowel syndrome (IBS) usually starts in early adulthood. It affects twice as many women as men. Approximately 10% to 20% of the population has IBS. No one knows what causes IBS. Some studies suggest that the nerves of the colon may be much more sensitive than usual in people with IBS. The normal movement of food and gas through the colon causes pain, intestinal spasms and an irregular pattern of bowel movements.