Gerry, if your description is accurate, the problem sounds like a lack of waterproofing. Cement board is not damaged by water, but it does not block it either (though I have met people who think it does). Geneally waterproofing is done with felt or poly behind the backer or with a product like NobleSeal over the cement bd. You can patch, this time putting building paper between the drywall and CBU, but the only long term fix is to tear it out and do a proper installation. Sorry the news isn't better. If you decide to do it over, look into a product called Dens-Shield by Georgia Pacific, it's much easier to work than CBUs and it's got a waterproofing mambrane built in.