A urinary tract infection uti is an infection in any part of your urinary system your kidneys ureters bladder and urethra most infections involve the lower urinary tract the bladder and the urethra women are at greater risk of developing a uti than are men infection limited to your bladder can be painful and annoying. 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. 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. Urinary tract infections utis national womens health network urinary tract infections utis uti an infection in the urinary system your kidneys ureters bladder and urethra most infections involve the lower urinary tract the bladder and the urethra urinary tract infections are more common in women