A urinary tract infection (UTI) is caused by bacteria invading the urinary system, which includes organs like the kidneys and bladder. A UTI most often develops in the bladder. (The bladder holds the urine until you decide to go to the bathroom.) But it can also affect the kidneys, ureters (the tube that moves urine from the kidney to your bladder), and the urethra (the tube that carries urine out of the bladder).