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 C.D.C.′s executive director, said during an unusual late-evening telephone news conference. That organization also reviewed the new guidelines, he said.

They are voluntary, not required by law.

Among other changes, the guidelines say that no skin should be left uncovered; that street clothes and shoes should be replaced with waterproof fabric and boots; and that every step of putting on and taking off equipment must be done under the eyes of a supervisor whose job is to prevent mistakes.