There are two reasons:
1. The H5N1 strain of highly pathogenic avian influenza (HPAI) virus has caused very severe illness or death in the people who have become sick with it.
2. Of more concern is the possibility that the virus will change (through a process scientists refer to as antigenic shift) into a form that could easily be passed from person to person. Since the human population would have no natural immunity against this strain, a large percentage of the population would be at risk of contracting the virus and becoming ill.