Nobody knows for sure, although there are many theories.
It seems most likely that the diagnosis of IC covers a number of different syndromes with similar symptoms. Usually the doctor diagnoses IC by first ruling out other possibilities such as bacterial cystitis or bladder tumors. Even without treatment, IC tends to go in cycles when it is better, worse and then better again. Today, there are several treatments and medications you can try. Using these, some patients with IC may eventually become symptom-free.