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.

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.

A child ill with fever and conjunctivitis in San Francisco tested positive for bird flu but had no known source of transmission.

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.