Avian influenza viruses are carried among birds but don’t usually infect humans. Of the few avian influenza viruses that have crossed the species barrier to infect humans, H5N1 has caused the largest number of detected cases of severe disease and death in humans. The H5N1 virus does not infect humans easily, and if a person is infected, it is very difficult for the virus to spread to another person. Nonetheless, there is concern that the H5N1 virus could mutate and gain the capacity to spread easily from one person to another. Because avian viruses do not commonly infect humans and there is little or no immune protection against them in the human population, H5N1 is monitored as a pandemic threat.

http://www.cdc.gov/flu/
http://www.who.int/csr/disease/influenza/en/