What you don't know can indeed hurt you. It can hurt you a lot. For example, as head of the Atlanta-based Centers for Disease Control and Prevention, the nation's top public health agency, Dr. Julie Gerberding was supposed to brief Congress this week about the health implications of global warming. But before she could testify, the guts of her prepared presentation - more than half its original 14 pages - were removed under orders from the Bush White House. Now, President Bush had every right to change that testimony.