Influenza A virus subtype H5N1 is a subtype of the Influenza A virus. Subtype H5N1 is commonly known as avian influenza or bird flu. H5N1 may cause more than one influenza pandemic as it is expected to continue mutating in birds. The dominant strain of HPAI A (H5N1) evolved creating the Z genotype. It has also been called Asian lineage HPAI/A/H5N1 which is divided into 2 antigenic clades. Clade 1 includes human and bird isolates from Vietnam, Thailand, and Cambodia and bird isolates from Laos and Malaysia. Clade 2 viruses were first identified in bird isolates from China, Indonesia, Japan, and South Korea before spreading westward to the Middle East, Europe, and Africa. Influenza A virus is a major public health threat. Novel influenza virus strains caused by genetic drift and viral recombination emerge periodically to which humans have little or no immunity, resulting in devastating pandemics. Influenza A can exist in a variety of animals; however it is in birds that all subtypes can be found. These subtypes are classified based on the combination of the virus coat glycoproteins hemagglutinin (HA) and neuraminidase (NA) subtypes. During 1997, an H5N1 avian influenza virus was determined to be the cause of death in 6 of 18 infected patients in Hong Kong. There was some evidence of human to human spread of this virus, but it is thought that the transmission efficiency was fairly low. HA interacts with cell surface proteins containing oligosaccharides with terminal sialyl residues. Virus isolated from a human infected with the H5N1 strain in 1997 could bind to oligosaccharides from human as well as avian sources, indicating its species jumping ability. H5 stands for the fifth of several known types of the viral protein hemagglutinin. This protein binds to sialic acid-containing receptors on the cell surface, bringing about the attachment of the virus particle to the cell. It plays a major role in the determination of host range restriction and virulence and is responsible for penetration of the virus into the cell cytoplasm by mediating the fusion of the membrane of the endocytosed virus particle with the endosomal membrane.