Avian Flu, Bird Flu, Influenza A virus subtype H5N1, also known as A(H5N1) or H5N1. Avian flu is any flu caused by a virus adapted to birds. (Also called "bird flu", "avian influenza" and "bird influenza".)

The only known such viruses are Influenza A viruses. Most people use "avian flu" to mean H5N1 which is the subtype of the species Influenza A virus that is a current pandemic threat.

Avian flu viruses are noninfectious for most species. When they are infectious they are usually asymptomatic, so the carrier does not have any disease from it.

Thus while infected with an avian flu virus, the animal doesn't have a "flu". Typically, when illness (called "flu") from an avian flu virus does occur, it is the result of an avian flu virus strain adapted to one species spreading to another species (usually from one bird species to another bird species).

So far as we know the most common result of this is an illness so minor as to be not worth noticing (and thus little studied). But with the domestication of chickens and turkeys, we have created species subtypes (domesticated poultry) that can catch an avian flu virus adapted to waterfowl and have it rapidly mutate into a form that kills in days over 90% of an entire flock and spread to other flocks and kill 90% of them and can only be stopped by killing every domestic bird in the area.

Until H5N1, this was basically the whole story of avian flu so far as anyone knew or cared (outside of the poultry industry). Now with H5N1, we have a whole new ballgame with H5N1 inventing new rules as it goes with behaviors never noticed before, and possibly never having occurred before.