Normally, UTI is prevented by acidic urine, complete bladder emptying, a competent ureterovesical junction to prevent urine backflow, and bacteriostatic properties of the urethra and bladder. The urinary tract above the urethra is sterile; therefore if bacteria ascends the urethra and colonizes in the bladder, an inflammatory process is initiated. The process of voiding routinely cleanses the bladder and urethra. Thus anything that interferes with this process, such as outflow obstruction, postponement of voiding, low fluid intake, vesicoureteral reflux, or diabetes mellitus, increases the risk of developing a UTI. Other causes of a UTI include catheterization, constipation (inhibits complete bladder emptying), poor perineal hygiene, and a neurogenic bladder. Sexually active adolescent girls are also prone to develop UTIs.

The catheterization procedure may introduce bacteria into the bladder because the process of catheterization causes irritation and minute scraping
of the urethra, creating a portal for bacterial entry. Additionally, an indwelling catheter prevents normal flushing of the urethra by urine, and bacteria may travel to the bladder through the catheter itself or via the exudate that collects between the outside of the catheter and the urethral walls. As bacteria adhere to the catheter, they produce a protective film that covers the surface of the catheter and protects the bacteria against antibiotic action, making eradication of the infection difficult.