A urinary tract infection (UTI) is an infection in any part of the urinary system. Most UTIs start in the lower urinary tract in the bladder or urethra. The urethra is the tube that carries urine out of the body. A UTI can also include an infection in the upper urinary system, including the kidneys.

There are different names for infections in different parts of the urinary system, including:

The infection may also occur in the tube connecting the bladder to the kidney. All of these infections are considered to be UTIs.

The Urinary Tract
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UTIs are caused by bacteria that most often come from the digestive tract or rectal area. The bacteria cling to the opening of the urethra and begin to multiply. If the infection is not treated right away, bacteria may move up the urinary system to the kidneys.

Most infections are caused by a bacteria that normally lives in the colon. The bacteria may move from the rectal area to the urethra.

UTIs can also be sexually transmitted. This type of infection usually does not spread past the urethra. Both partners need to be treated.