The H5N1 influenza virus is a strain of the A subtype of influenza. This virus primarily infects avian species but has been known to periodically mutate and jump into the human population. Recent outbreaks of H5N1 influenza, particularly an outbreak in 2008, have caused great concern, as this strain of influenza has the potential to become a very serious epidemic disease if it is allowed to spread widely through the human population. A particularly dangerous variety of this virus, referred to as the highly pathogenic strain, is an object of particular concern, both because of its ability to decimate populations of domesticated birds and because it is more apt to produce serious symptoms when it infects humans.
H5N1 influenza is commonly found among wild birds. In these animals, it is typically either asymptomatic or the cause of only very mild symptoms. The virus spreads easily in the wild through both animal to animal contact and through a variety of environmental vectors. It can easily spread to populations of domesticated birds as well, which are typically more vulnerable to the disease, particularly highly-domesticated breeds. The highly pathogenic variety of H5N1 influenza spreads with equal ease through both avian populations but has more serious effects on its hosts, often leading to severe illness or death.
Avian influenza poses major risks to worldwide poultry production but also poses serious human health risks. Although strains of influenza tend to remain in one single species or type of species, they do mutate with some regularity and can jump from species to species. One of the most serious outbreaks of influenza in recorded history, the pandemic of 1918, often called the Spanish influenza, was of avian origin. This influenza pandemic killed millions of people in a world that lacked air travel and other common paths for disease transmission that are available in the modern world.
All varieties of influenza, including H5N1 influenza, occasionally infect humans who have extensive contact with animals carrying the virus. This has happened repeatedly in recent years with the H5N1 virus. This strain of influenza has not yet managed to mutate in such a way as to be able to reliably spread from human host to human host, however. Until such a mutation takes place, the H5N1 influenza will not be able to cause a pandemic in the human population. Isolated incidents of human to human transmission have been reported, however, and such mutations have occurred regularly in the past, so a pandemic is far from impossible.