A urinary tract infection (UTI) is a common infection that can involve bladder, kidneys, the ureters, the tubes that connect the bladder and the kidneys, and the urethra, the tube which carries urine out of the body from the bladder.

In most cases, bacteria first begin growing in the urethra. An infection limited to the urethra is called urethritis. From there bacteria often move on to the bladder, causing a bladder infection (cystitis). If the infection is not treated promptly, bacteria may then go up the ureters to infect the kidneys (pyelonephritis).

UTIs are the second most common infection after respiratory infections. It is estimated that each year, 8 to 10 million people in the United States have a UTI.