Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Bladder Infection (Cystitis)

What Is It?

A bladder infection, also called cystitis, is caused by an abnormal growth of bacteria inside the bladder, the balloon-like organ that stores urine. Bladder infections are one of the most common bacterial infections to affect humans, with up to one-third of all females having at least one infection at some point in their lives.

Bladder infections are classified as either simple or complicated. Simple bladder infections affect only healthy women with normal urinary systems. Bladder infections are rare in men who are otherwise healthy, so men are included in the complicated category with members of both sexes who have abnormal urinary systems.

• Simple bladder infections — Simple bladder infections develop when bacteria migrate into the bladder. Because of the structure of the female urinary system, women are far more likely than men to get these infections. In women, the opening to the urethra (where urine comes out) is close to the rectum. Therefore, bacteria can migrate from the rectum, where bacterial counts are high, to the area around the vagina and urethra. From there, it is a short trip (4 centimeters, or less than 2 inches) through the urethra to the bladder. Sexual intercourse can propel these bacteria into the bladder, so there is an increased risk of bladder infections in sexually active women. Wiping toilet tissue from back to front after having a bowel movement also can transfer bacteria from the rectum to the urethra. For this reason, women should always try to wipe front to back.

• Complicated bladder infections — Bladder infections are classified as complicated when they affect people with an abnormal urinary system that makes these infections more difficult to treat. All bladder infections are considered complicated when they affect men, because the long male urethra should prevent bacteria from getting into the bladder. However, if the normal flow of urine is obstructed or urine is retained in the bladder, bacteria will multiply there, increasing the risk of infection. One reason urine may be retained in the bladder is nerve damage, either from an injury, such as a spinal cord injury, or because of a disease, such as diabetes. The most common reason for obstruction of urine flow in men is an enlarged prostate, common in men older than 50. Patients with indwelling urinary catheters (a tube inserted into the urethra to drain urine) also have high rates of bladder infections because the bacteria climb along the wall of the catheter to the bladder.