Urinary tract infections are caused by organisms such as bacteria overcoming the body's defenses in the urinary tract. They can affect the kidneys, bladder, and the tubes that run between them. The urinary tract can be divided into the upper urinary tract and the lower urinary tract. The upper urinary tract consists of the kidneys and the ureters, and the lower urinary tract consists of the bladder and the urethra. Most UTIs are not serious, but some can lead to serious problems, particularly with upper UTIs. Recurrent or long-lasting kidney infections can cause permanent damage, and some sudden kidney infections can be life-threatening, particularly if bacteria enter the bloodstream in a condition known as septicemia.