Avian flu or bird flu refers to strains of the influenza virus which circulate among bird populations - in particular, among chickens. In general terms, there is nothing particularly distinctive about bird flu compared to human flu other than its usual victims, and on occasion, the virus does jump the species barrier, moving from humans to birds or vice versa.
Influenzavirus is actually a genus of different viruses, which infect several different animal reservoirs in addition to human beings. Some flu viruses, for example, target dogs (dog flu) and horses (equine flu). However, these are relatively uncommon compared to the more common flu infections amidst humans and the other two sizeable animal reservoirs, pigs (swine flu) and birds (avian flu). Identifiable subtypes of bird flu have been identified in chickens as well as in ducks, turkeys, and occasionally other bird species. In most but not all cases, bird flu does not actually cause serious symptoms within the bird species it infects.
In recent years, most of the interest in bird flu has related to fears that a strain of one particular subtype, H5N1, will eventually jump the species barrier between chickens and humans. H5N1 spread rapidly in domesticated chicken populations around the world during the 1990s and attracted considerable attention because, unlike most bird flu strains, it often proved highly deadly to chicken flocks it infected. H5N1 has incited further fears because of speculation that if it mutates into a form capable of infecting humans, the unusually deadly strain would result in a worldwide pandemic as deadly or even more deadly than the 1918-1919 Spanish flu pandemic, which killed over twenty million people.
It is important to recognize the danger of flu viruses jumping between host species. For example, the current H1N1 pandemic is conventionally referred to as "swine flu," but the virus in question actually carries genetic remnants of strains which previously jumped from humans to pigs, and also from birds to pigs (it is thus, in a sense, human-swine-avian flu and not just swine flu). The H1N1 pandemic made its final jump from pigs to humans rather than from birds to humans, but does illustrate the danger posed by such transfers.
Nevertheless, it is equally important to recognize that the risk in any given year, and in any given population, is actually quite limited. In the vast majority of cases, flu viruses do not transfer between individual birds and individual humans. Moreover, on those small number of occasions on which they do, the virus usually proves incapable of being transmitted to infect any other humans. For example, a small number of humans - mostly agricultural workers - have been infected with H5N1 flu to date, and the mortality rate for identified cases is as high as 60%. However, none of these cases are known to have resulted in transmission from the original infected human case to other human beings.