Influenza A virus is a major public health threat, killing more than 30,000 people per year in the USA. 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, but 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. HA interacts with host cell surface proteins containing oligosaccharides with terminal sialyl residues. Its extracellular region has two domains (HA1 and HA2); HA1 is cleaved from the main hemagglutinin protein by the host immune system. During 1997, an H5N1 avian influenza virus was determined to be the cause of death in 6 of 18 infected patients in Hong Kong. This more recent virulent strain of H5N1 is now seen in Africa and Europe, as well as in Southeast Asia. There is some evidence of human to human spread of this virus, but it is thought that the efficiency of this type of transmission is low. 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.