Bacteria that enter the urethra and travel up the urinary tract are the usual cause of urinary tract infections (UTIs). Bacteria that normally live in the large intestine and are present in feces (stool) are the most common source of infection. Sexual intercourse may introduce bacteria into the urinary tract, especially in women. Catheters, which are small, flexible tubes inserted into the bladder to allow urine to drain, are a common source of bacterial infection in people who are hospitalized or live in long-term care facilities.

Occasionally, bacteria traveling through the blood or lymph system cause kidney or bladder infections.

Kidney stones, an enlarged prostate in men, and structural problems in the urinary tract can contribute to UTIs by limiting the body's ability to eliminate urine completely.