One of the ugliest diseases ever. In Western Civilization, we want childbirth to be a time of joy and contentment and fulfillment. That's a happy semi-myth, but it sure did happen for me. Breast-feeding a new baby makes "the world go away" in the most pleasant way.
PPD, though rare, as contrasted with "baby blues," which is common, can be devastating. Brooke Shields did a book about her PPD. Get the word out, Brooke. It is easily cured.