The urinary tract refers to the connected system of organs
through which urine flows on its way out of the body. This
includes the two kidneys, the two ureters (tubes that connect
the kidneys to the bladder), the bladder, and the urethra (the
tube from the bladder through which urine leaves the body). A
urinary tract infection can affect any or all of these
structures, although a kidney infection (pyelonephritis) is
generally discussed as a separate condition. A urinary tract
infection usually occurs when bacteria on the skin or in the
genital or rectal area enter the urinary tract via the urethra.
If conditions are right, these bacteria can multiply within the
urethra and bladder, causing infection. Sometimes infections
begin during a medical procedure that requires placement of a
catheter, a thin flexible tube that is inserted through the
urethra and into the bladder, to drain urine. Organisms can
“climb” up the catheter into the bladder, resulting in a urinary
tract infection. Sexual intercourse is another