Each state has different rules as to how long a creditor can sue for a medical bill, which is normally five years in Nebraska and Iowa from the last date payment made.
Outstanding medical bills that have gone on to a collection agency typically only stay on your credit report for 7 years, however, that does not mean that those medical bills are not still due. The bills are only removed (legally not enforceable) if you pay them in full, they are removed by the provider, or you file for bankruptcy.
Also, as mentioned above, if a creditor waits too long to file a lawsuit, typically after five years from the date of last making a payment, then the statute of limitations provides you with a legal defense.