Influenza is a highly contagious acute viral infection caused by influenza viruses. All influenza viruses are part of the orthomyxoviridae family of viruses. Avian influenza refers to those influenza strains that can cause illness in birds through bird to bird transmission, while human influenza refers to strains that affect people, through person to person transmission.

Transmission of avian influenza to people can occur, but it is rare as the transmission mechanism is inefficient. Thus even when there is an outbreak of avian influenza in birds, the risk of transmission to people is believed to be small.

While influenza viruses can be classified generically as belonging to a certain family (influenza type A, B or C viruses), within a species, unique serotypes are identified with letter/number combinations, such as H1N1, H3N2, etc, which is often how they are referred to in the media. Some important strains that caused previous influenza outbreaks are shown below.

Influenza A viruses are believed to live in a large number of warm-blooded animals including birds, pigs, horses, and people. All subtypes of Influenza are found in birds, making them the natural carrier for all influenza strains, not just avian influenza. Strains present in one species can interact with a strain from another species, causing a new version or serotype. This type of evolution, called antigenic shift, is responsible for the constant evolution of influenza.

One of the main concerns with Influenza is that it is highly contagious between people and between animals. Also, some people and animals can carry influenza viruses without developing symptoms of illness, so not every person or animal carrying the virus develops the disease. Farms that raise animals, especially bird and poultry farms, have a wide range of risks of pathogen transmission, including pathogens such as avian influenza. This risk extends from the farm level, where birds/poultry are raised prior to processing, to the bird/poultry processing facilities, to the facilities that receive the processed poultry including restaurants, grocery stores, hotels, and other facilities that receive raw birds and poultry.