Most UTIs stay in the bladder,the pouch-shaped organ where urine is stored before it passesout of the body. If a UTI is not treated promptly, the bacteria can travel up to the kidneys andcause 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.