Fifteen different types of hemagglutinin, a protein embedded in the surface of the influenza virus, have been identified in influenza viruses, tagged as H1, H2 and H3, etc. H1, H2 and H3 have all been found in human viruses, the others are principally found in avian reservoirs. The binding of hemagglutinin, to glycoproteins on a cell's surface prompts the cell to engulf the virus, thus allowing the virus to hijack cellular mechanisms for its own reproduction. Chemical and biological studies suggest that hemagglutinin binds in a helical form, while it is generally in a random coil configuration in solution. Thermodynamics and statistical mechanics can be used to study the shift. pH appear to be a critical factor in the helix-coil equilibrium for this protein.
Hemagglutinin is often used as the immunizing agent in flu vaccines. The 1957 flu virus that was inadvertently included in a set of controls recently sent to 5000 labs around the world was type H2N2 (N refers to the type of neuroamidase - another protein found on the surface of the virus). The avian influenza circulating in Asia recently is type H5. While occasionally humans have contracted this form of the virus, it has not spread person to person, however the lack of general human immunity to H5 worries public health officials. The hemagglutinin found in the influenza virus from the 1918 pandemic is similar to avian forms of the protein. Over 12,000 people died from contracting the 1918 virus in Philadelphia alone in a one month period. World War I was raging in Europe, but the total US combat deaths in that war, staggering as they were at the time (53,513) were dwarfed by the number of Americans who died in the 1918 epidemic: 850,000.
Microscopic terrorists hijacking our cells should worry us at least as much as human terrorists - and indeed homeland security considers a flu pandemic to be a potential catastrophe.