Casuative agent of Bird flu (avian influenza).

All influenza viruses are divided into three types - A, B or C - depending on the virus' structure.
Type A is responsible for lethal influenza pandemics, whereas
Type B causes smaller, localized outbreaks.
Both types A and B are also responsible for the seasonal flu that affects many people every year. Less common and more stable than other strains, type C has milder symptoms.
Influenza B and C are usually found only in humans. But type A influenza infects both people and animals, including birds, pigs, horses, whales and seals.
Numerous influenza A subtypes exist, and they can combine to form even more subtypes, some of which normally are specific to a single species. At least 15 flu subtypes affect birds, the most virulent of which is H5N1. Until recently, avian subtypes have rarely been found in humans or in animals other than pigs.
Type A influenza viruses are further divided into strains, which are constantly evolving. And it is exactly this - the ability of influenza viruses to change their genetic makeup and to swap genes indiscriminately - that makes them so unpredictable and potentially deadly.