Urinary tract infections (UTI) are caused by bacteria that enter the urinary tract through the urethra, the tube that empties the bladder during urination. Bacteria may gain access to the urethra in a number of ways. They can be transmitted during sexual intercourse or occur when wiping from the anus toward the vagina after a bowel movement or when changing a baby’s diapers. The use of catheters or tubes in the urethra can also introduce bacteria into the area. This is a common cause of hospital-acquired infections.  

Certain factors are associated with a greater risk of developing UTIs. For example, a person with a family history of UTI may be more likely to have UTIs. Additional risk factors include:

• Gender. UTIs occur more often in women than men. This may be due to the shorter length of the urethra in females (bacteria have less distance to travel), as well as the proximity of the urethra opening to the vagina, which naturally harbors many bacteria, and to a lesser degree, to the anus. As many as one in five women will experience a UTI at some point in their lives, according to the National Kidney Foundation.
  
  
• Menstrual status. Postmenopausal women have reduced levels of estrogen, which can lead to a thinning of the vaginal and urethral tissues. This in turn reduces the body’s resistance to bacterial infections such as UTIs.
  
  
• Certain birth control methods. Diaphragms can harbor bacteria if not properly cleaned, increasing the risk of bacterial infections, including UTIs. In addition, diaphragms may put pressure on the urethra, reducing urine flow and allowing the buildup of bacteria. Most spermicides kill beneficial bacteria normally found in the vagina and have no effect on bacteria that can lead to a UTI (e.g., Escherichia coli).
  
  
• Urinary obstruction. Pressure or blockage anywhere in the urinary tract (e.g., due to kidney stones, tumor, enlarged prostate) can interfere with the normal flow of urine. This may allow the buildup of bacteria within the tract, leading to a UTI.
  
  
• Infrequent urination. When urine remains in the bladder for long periods of time, it allows any bacteria in the area to multiply, possibly leading to a UTI. This may occur if people do not drink enough fluids. Infrequent urination is common among older adults, young children, the seriously ill and disabled people.
  
  
• Structural abnormalities. Abnormal structure of the urinary tract due to a birth defect may increase the risk of developing a UTI. Repeated UTIs in very young children may indicate such an abnormality.