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

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.

The current public-health risk is low, but the CDC is watching the situation carefully and working with states to monitor people with animal exposure.