The bladder holds urine produced by the kidneys. The bladder and the urine it holds are normally free from bacteria and other organisms. A urinary tract infection occurs when organisms are introduced into the bladder. Bacteria from the skin may enter through the urethra. The urethra is the tube that carries urine from the bladder to the outside of the body.
Organisms can also enter the bladder on urinary catheters .
Bacteria in the bladder are usually flushed out when the person urinates. If bacteria multiply faster than they can be removed, a UTI results. UTIs are more common in women than in men. Women have a shorter urethra which allows easier access for bacteria. The most common cause of a UTI is a bacteria called E. coli.