The opening of the urethra is at the end of the penis in men and in front of the vagina in women. When this happens, the bacteria cause infection and inflammation of the bladder. In other words, they multiply, causing redness, swelling, and pain. Bladder infection, also called cystitis , is the most common urinary tract infection. If the bacteria travel upward from the bladder through the ureters and reach the kidneys, you may develop a kidney infection, also known as pyelonephritis .
Kidney infections are much less common but often more serious than bladder infections.