A urinary tract infection (UTI) is an infection caused by bacteria in part of the urinary tract. In the lower urinary tract, it is known as a simple cystitis (a bladder infection). In the upper urinary tract, it is known as pyelonephritis (a kidney infection). Symptoms from a lower urinary tract infection include painful peeing and either frequent peeing or wanting to pee (or both). Symptoms of a kidney infection also include fever and side and back pain. In old people and young children, the symptoms are not always as clear. The main cause for both types is the bacteria "Escherichia coli." Other bacteria, viruses, or fungus may be the cause in rare cases.