Avian influenza, more commonly known as bird flu is an infectious disease of birds caused by type A strains of the influenza virus. There are approximately fifteen subtypes of the influenza viruses. Influenza A (H5N1) is a subtype of the Type A influenza virus and the most common type influenza. Bird flu viruses infect birds, including chickens, other poultry and wild birds such as ducks. Most bird flu viruses can only infect other birds. Wild waterfowl are the main carriers of this virus and can be responsible for the primary introduction of infection into domestic poultry. When avian influenza spreads to poultry or other birds, it can cause more severe diseases. Bird flu can pose health risks to humans. The first case of a bird flu virus infecting a person directly, H5N1, was in Hong Kong in 1997. Since then, the bird flu virus has spread to birds in countries in Asia, Africa and Europe. Among domestic poultry species, turkeys are more commonly infected than chickens. All birds are thought to be susceptible to infection with avian influenza, though some species are more resistant to infection.
Avian influenza in humans is rare and the most common route of infection is via direct or indirect contact with secretions (nasal, oral or fecal) from infected poultry.Some rare cases of human infection with avian influenza A viruses have been reported. Transmission from human-to-human, if it exists, is extremely rare. However, avian influenza viruses have the potential to mutate or reassert itself and become pandemic viruses; those that can be readily transmitted between humans and those for which the population has little immunity. Human clinical illness from infection with avian influenza A viruses have reported signs and symptoms which range from eye infections (conjunctivitis) to influenza-like illness symptoms (e.g., fever, cough, sore throat, muscle aches) to severe respiratory illness (e.g. acute respiratory distress, pneumonia or viral pneumonia) sometimes accompanied by nausea, diarrhea, vomiting and neurologic changes and even death. If these viruses spread throughout the world, the disease caused by them would be called pandemic influenza.