People hospitalized for flu should be tested for bird flu within 24 hours, the U.S. Centers for Disease Control and Prevention said on Thursday, in an expansion of the agency's efforts to tackle increasing infections in humans.

CDC officials say medical professionals are seeing more patients whose illness cannot be traced back to an infected animal or bird.

CDC officials say they extended the guidance now because they are seeing more H5N1 patients whose illness they cannot track back to an infected bird or cow.