A urinary tract infection is what happens when bacteria (germs) get into the urinary tract (the bladder) and multiply. The result is redness, swelling and pain in the urinary tract (see diagram). Most UTIs stay in the bladder, the pouch-shaped organ where urine is stored before it passes out of the body. If a UTI is not treated promptly, the bacteria can travel up to the kidneys and cause a more serious type of infection, called pyelonephritis (pronounced pie-low-nef-rightiss). Pyelonephritis is an actual infection of the kidney, where urine is produced. This may result in fever and back pain.