Avian influenza, or so called “bird flu,” is a disease caused by the avian influenza viruses. The viruses naturally lie in digestive tracts of birds and do not cause obvious signs of infection. Avian influenza viruses can be divided into highly pathogenic avian influenza virus and low pathogenic avian influenza virus. Birds infected by highly pathogenic avian influenza virus are characterized by their untidy feathers, loss of appetite, failure to lay eggs, and purple cockscombs. Poultry infected by highly pathogenic influenza virus have a lethality rate of 80% or higher.
Currently the H5N1 virus has crossed the species barrier to infect human beings, and may cause a lethal global pandemic after transforming into a virus with person-to-person transmission. As a result, the flu pandemic is also called the Killer Flu or Super Flu, which is most likely to be caused by H5N1.