Untreated Urinary Tract Infections: Bladder & Kidney Infections. Infection occurs when the bacteria cling to the opening of the urethra and multiply, producing an infection of the urethra, called urethritis. The bacteria often spread up to the bladder, causing a bladder infection, called cystitis. Untreated, the infection can continue spreading up the urinary tract, causing infection in the kidneys, called pyelonephritis. Pyelonephritis can also occur without a preceding bladder infection.