WORLD HEALTH ORGANIZATION

http://www.who.int

Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus.

Of all influenza viruses that circulate in birds, the H5N1 virus is of greatest present concern for human health for two main reasons. First, the H5N1 virus has caused by far the greatest number of human cases of very severe disease and the greatest number of deaths. It has crossed the species barrier to infect humans.

Extensive studies of the human cases determined that direct contact with diseased poultry was the source of infection.

All evidence to date indicates that close contact with dead or sick birds is the principal source of human infection .

One feature seen in many patients is the development of manifestations in the lower respiratory tract early in the illness. Many patients have symptoms in the lower respiratory tract when they first seek treatment.

Id-Jet