Federal officials announced new guidelines on Monday evening for the protection of hospital workers caring for patients infected with Ebola--guidelines that might have prevented the infection of two nurses had they been in place a month ago.The new guidelines, from the Centers for Disease Control and Prevention, follow broad revisions announced just last week. The new recommendations provide considerably more detail, however, and have been reviewed by specialists at American hospitals that have successfully cared for Ebola patients. The procedures are based on the very strict protocols used for years by Doctors Without Borders, Dr. Thomas R. Frieden, the CDC's executive director, said during an unusual late-evening telephone news conference.