AVIAN influenza, known informally as avian flu or bird flu, is a variety of influenza caused by viruses adapted to birds.

The most widely-quoted date for the beginning of recorded history of avian influenza - initially known as fowl plague - was in 1878 when it was differentiated from other diseases that caused high mortality rates in birds.

It is similar to swine flu, dog flu, horse flu and human flu as an illness caused by strains of influenza viruses that have adapted to a specific host.

Out of the three types of influenza viruses (A, B, and C), influenza A virus has a natural reservoir almost entirely in birds.

Avian influenza, for most purposes, refers to the influenza A virus.

Though influenza A is adapted to birds, it can also adapt and sustain person-to-person transmission.

Recent influenza research into the genes of the Spanish flu virus shows it to have genes adapted from both human and avian strains.

Pigs can also be infected with human, avian and swine influenza viruses, allowing for mixtures of genes to create a new virus.

This cocktail can cause a virus subtype that most people have will have little to no immune protection against.

The most well-known bird flu strain was first isolated from a farmed goose in Guangdong Province, China, in 1996, and also has low pathogenic strains found in North America.