Cystitis occurs when the normally sterile lower urinary tract (urethra and bladder) is infected by bacteria and becomes irritated and inflamed. It is very common, occurring in more than 6 million Americans a year.

The condition frequently affects sexually active women ages 20 to 50 but may also occur in those who are not sexually active or in young girls. Older adults are also at high risk for developing cystitis, with the incidence in the elderly being much higher than in younger people.

Cystitis is rare in males. Females are more prone to the development of cystitis because of their relatively shorter urethra -- bacteria do not have to travel as far to enter the bladder -- and because of the relatively short distance between the opening of the urethra and the anus.

More than 85% of cases of cystitis are caused by Escherichia coli, a bacterium found in the lower gastrointestinal tract. Sexual intercourse can increase the risk of cystitis because bacteria can be introduced into the bladder through the urethra during sexual activity. Once bacteria enter the bladder, they normally are removed through urination. When bacteria multiply faster than they are removed by urination, infection results.

Risks for cystitis include obstruction of the bladder or urethra with resultant stagnation of urine, insertion of instruments into the urinary tract (such as catheterization or cystoscopy), pregnancy, diabetes, HIV, and a history of analgesic nephropathy or reflux nephropathy.

The elderly of both sexes are at increased risk for developing cystitis due to incomplete emptying of the bladder associated with such conditions as benign prostatic hyperplasia (BPH), prostatitis and urethral strictures. Also, lack of adequate fluids, bowel incontinence, immobility or decreased mobility and placement in a nursing home, all put people at increased risk for cystitis.