UTIs are caused by bacteria. The bacteria invade the urinary system and multiply. The infection can occur in any part of the urinary system, but usually starts in the urethra. The urethra is a tube that carries the urine out of the body.

The Urinary Tract
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In most cases, bacteria begins growing in the urethra. The bacteria comes most often from the digestive tract and rectal area. They cling to the opening of the urethra and begin to multiply. An infection only in the urethra is called urethritis. From there, bacteria move to the bladder, causing a bladder infection. If the infection is not treated right away, bacteria may then go up the ureters. Ureters are two tubes that carry urine from the kidneys to the bladder. From there the infection can continue into the kidneys ( pyelonephritis).

Most infections are caused by one type of bacteria called E. coli. E. coli normally lives in the colon. In women, since the rectum and urethra are fairly close to each other, the bacteria can move into the urethra. This makes women more prone to UTIs than men.

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