Most urinary tract infections (UTIs) start in the lower urinary tract, which is made up of the urethra and bladder. Bacteria from the bowel live on the skin near the anus or in the vagina. These bacteria can spread and enter the urinary tract through the urethra. If they move up the urethra, they may cause a bladder infection (called cystitis). Bacteria that have infected the bladder may travel to the upper urinary tract, the ureters and the kidneys. An infection of the kidneys is called pyelonephritis. An upper urinary tract infection may cause a more severe illness than a lower urinary tract infection.
Women are more likely than men to get UTIs because the urethra is shorter in a woman than in a man. In women, the bacteria can reach the bladder more easily.