The other 65 known cases in the U.S. have all had been mild, though a 13-year-old girl in Canada with H5N1 also had severe illness.

The patient, who was older than 65 and had underlying medical conditions, was hospitalized Dec. 18 in critical condition with severe respiratory illness.

The first U.S. bird flu death has been reported. Louisiana health officials announced the death Monday of a person who had been hospitalized with severe respiratory symptoms.