The outbreak of bird flu in the U.S. has alarmed researchers and prompted new efforts to track the virus that's already killed millions of birds from Europe to Antarctica. As H5N1 continues to jump into mammals, many scientists are concerned that we're not watching closely enough as this virus spreads. William Brangham reports. A warning: This story contains scenes of animals in distress. PBS NewsHour is supported by - https://www.pbs.org/newshour/about/funders