a urinary tract infection (uti) is an infection caused by bacteria in part of the urinary tract. in the lower urinary tract, it is simple cystitis (a bladder infection). in the upper urinary tract, it is a kidney infection. symptoms from a lower urinary tract infection include painful peeing and frequent peeing or wanting to pee (or both). symptoms of a kidney infection include fever and side and back pain. in old people and young children, the symptoms are not always so clear. the main cause for both types is the bacteria "escherichia coli." other bacteria, viruses, or fungus may be the cause in rare cases.