A UTI is an infection in the urinary tract. Infections are caused by microbes-organisms too small to be seen without a microscope. Bacteria are the most common cause of UTIs. Normally, bacteria that enter the urinary tract are quickly removed by the body before they cause symptoms. But sometimes bacteria overcome the body's natural defenses and cause infection. The urinary tract is the body's drainage system for removing wastes and extra water. The urinary tract includes two kidneys, two ureters, a bladder, and a urethra. The kidneys are a pair of bean-shaped organs, each about the size of a fist. They are located below the ribs, one on each side of the spine, toward the middle of the back. Every minute, your kidneys filter about 3 ounces of blood, removing wastes and extra water. The wastes and extra water make up the 1 to 2 quarts of urine you produce each day. The urine travels from the kidneys down two narrow tubes called the ureters. The urine is then stored in a balloonlike organ called the bladder and emptied through the urethra, a tube at the bottom of the bladder. This publication by the National Institutes of Health (Publication No. 12-4807) provides information on the causes, signs and symptoms, diagnosis, and treatment of Urinary Tract Infections.