A urinary tract infection (UTI) is an infection of any part of the urinary system (including the kidneys and bladder). Urinary tract infections are more common in girls than in boys.
UTIs are usually caused by bacteria which infect the urinary tract. An infection can occur anywhere along the urinary tract, but the lower part - the urethra and bladder - is most commonly involved. This is called cystitis.
If the infection travels up the ureters to the kidneys, it's called pyelonephritis and it's generally more serious. Although bacteria are not normally found in the urine, they can easily enter the urinary tract from the skin around the anus. Intestinal bacteria E. coli is the most frequent cause of urinary tract infection.