A urinary tract infection is an infection involving the organs that produce urine and carry it out of the body. These structures include the kidneys, ureters (long, slender tubes connecting the kidneys with the bladder), bladder and urethra. Doctors often divide urinary tract infections into two types, lower tract infections and upper tract infections:
Lower tract infections — Infection of the bladder is called cystitis (bladder infection). Bacteria normally found in the intestine are the main cause of lower urinary tract infections. These bacteria spread from the anus to the urethra and bladder, where they grow, invade the tissue and cause infection.
Upper tract infections — These involve the ureters and kidneys. These infections are called pyelonephritis or kidney infections. Upper urinary tract infections usually occur because bacteria travel up from the bladder into the kidney. Sometimes, they occur when bacteria travel from other areas of the body through the bloodstream and settle in the kidney.
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