The cause of IBS is not known although they are finding some genetic clues. Food and stress are triggers for many-different foods for different people-lactose, fructose, gluten, fat but many people who don't have IBS and can't digest/absorb these foods might get D or gas without the discomfort and pain. It is ultimately largely the hypersensitivity of the bowel which distinguishes IBS. Some people develop this hyperensitivity after a bacterial or viral illness but not everyone with such illesses develop this hypersenitivity. So it is all up in the air. So that is it in a paragraph.