There are many different subtypes of type influenza viruses. These subtypes differ because of certain proteins on the surface of the influenza A virus (hem agglutinin [HA] and neuraminidase [NA] proteins). There are 16 different HA subtypes and 9 different NA subtype of flu A viruses, which can make 144 combinations of HA and NA proteins possible. Each combination is a different subtype. All known subtypes of flu A viruses can be found in birds. A/H5N1, also called the H5N1 virus, is a subtype of the Influenza A virus that mainly occurs in birds. It was first isolated from birds in South Africa in 1961. H5N1 virus is very contagious and fatal among birds; it has also caused some human infections since 1997. However, there is no sufficient information of efficient human-to-human transmission of H5N1.