Most infections begin in the urethra, the tube that drains the bladder. It is theorized that because in women the opening of the urethra is in close proximity to the anus and vagina in women, organisms can more readily move from these openings to the urethra. This is said to account for the higher infection rate in women. In women the risk of UTI increases with sexual activity and age.
Post-menopausal women may experience bladder or uterine prolapse or a shifting of these structures from their normal position. The shift can lead to incomplete emptying of the bladder and create conditions conducive to bacterial colonization. Postmenopausal women also experience changes in hormone production, particularly estrogen, which can alter vaginal flora, the good organisms that populate the vagina and fight bacteria.
Other risk factors for UTIs are obstructions in the urinary tract such as kidney stones. Poor bladder emptying and bladder control in the elderly put them at risk. In men an enlarged prostate may impede the flow of urine and increase risk. People who have catheters placed for diseases or surgical procedures are at risk despite extraordinary sanitization procedures employed during catheter placement and maintenance. Disorders such as diabetes that alter or weaken the immune system raise the risk of UTI by lowering natural resistance.
Several studies have suggested that women who use a diaphragm have a higher incidence of UTIs than those who use other means of birth control. In some women, intercourse may trigger the onset of a UTI although the reason for this has yet to be determined.