A rare digestive disorder that primarily affects females and typically becomes apparent during middle age. The disorder is characterized by inflammatory changes of the mucous membranes (mucosa) of the colon (colitis) and abnormal accumulation (excessive deposition) of the protein collagen beneath the surface (epithelial) layer of the mucosa (thickened subepithelial collagenous bands). The colon is the major portion of the large intestine. The medical literature often refers to collagenous colitis as a form of "microscopic colitis," since evidence of inflammation and other abnormalities may only be confirmed through microscopic (i.e., histologic) examination of multiple tissue samples.