It is considered a rare disease with only 700,000 cases in the US, but I think this is an incorrect number. I think that many more people have it but doctors are not testing them properly and they are just being given antibiotics and being told to go away. I would estimate that 1 in 20 women have a painful bladder issue and 1 in 50 men. It is supposed to affect women in their 40s, but I know 4 other women who all were diagnosed in their 20s. I had symptoms at 7. So, while the governmental agencies say that this is a condition that doesn't effect a lot of people, I think that it does but few people want to talk about their bladders.
However, it does affect women more than men and adults more often then children.