Unpaid medical bills do not go on your credit report unless the hospital or doctor to whom you owe the money subscribes to at least one of the three major credit reporting agencies (Experian, Equifax or Trans Union) or the debt is turned over to a collection agency. Moreover, if you dispute the validity of the debt within 30 days of receiving a payment notice, a creditor or debt collector cannot place the account on your credit report without a notation that you are disputing the debt.
Once a medical bill is recorded on a credit report it still can be removed if you agreed to pay the debt and negotiated to have the account removed from your credit report. Be sure to get any such agreements in writing, however, as otherwise it will not be legally binding. Do not only rely on the debt collectors oral promise that the account will be removed!