Most cystitis is caused by the migration of E. coli bacteria (which inhabit the digestive tract) from the anus into the urinary system via the urethra (the tube that carries urine from the bladder to the outside). Once in the bladder, the bacteria multiply.
Women are more prone to UTIs than men due to the proximity of the female urethra to the anus, and the relatively short length of the female urethra. Hormonal factors also contribute, making the female bladder a more hospitable environment for the bacteria during pregnancy and some stages of the menstrual cycle and after menopause.
Some women contract the infection following sexual intercourse, which can cause bacteria to be pushed into the bladder via the urethra.
These factors combine to mean that many women experience urinary discomfort at least once a year, and some suffer from recurrent episodes of cystitis.
Interstitial cystitis is a form of cystitis in which the symptoms of infection are present, but there is no detectable infection. This is a more difficult form of the condition to manage, and its cause is unknown, however family history, infections and allergies have all been suggested as possible causes or contributing factors.