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.

As the Centers for Disease Control and Prevention (CDC) explains, bird flu is a disease caused by the influenza A virus. At the same time, recent CDC data shows that seasonal influenza A is rising across the U.