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

The Centers for Disease Control and Prevention issued an advisory Thursday urging health-care workers treating patients hospitalized with the flu to perform an additional test for bird flu within 24 hours of admission.

By Leah Douglas and Tom Polansek (Reuters) -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 ongoing infections in humans.