Cystitis is a lower urinary tract infection involving inflammation of the bladder or urethra (the tube that transports urine out of the bladder). Cystitis is common in women but rare in men. Cystitis occurs when bacteria pass up along the urethra and enter and multiply within the bladder. As the urethra is much shorter in women than men, the passage of bacteria up the urethra and into the bladder is much easier.