Influenza A viruses infecting humans are responsible for a gamut of illnesses ranging from inapparent infections to pneumonia and severe acute respiratory syndrome. The typical infection one associates with seasonal influenza is a mild, self-limited febrile syndrome. As with other respiratory viruses, influenza viruses can cause more severe infections in infants, the elderly, and immunodeficient persons. In those individuals whose immune system is compromised or not fully developed, influenza-associated disease will often lead to severe viral pneumonitis or be complicated by bacterial superinfection, leading to pneumonia and sepsis. Seasonal influenza also occasionally results in neurologic complications. Even in non-pandemic years, more than 40,000 deaths annually are attributable to influenza infections in the United States (Dushoff et al., 2006). Much more rarely, humans can become infected by viruses that have spread directly from wild birds to domestic poultry. Such “avian influenza” can range in severity from mild conjunctivitis through the rapidly lethal disease seen in persons infected with the recently emerged Southeast Asian H5N1 virus. However, the potential for such highly virulent, lethal viruses to spread globally, as in the 1918 “Spanish Flu” pandemic, has resulted in a tremendous effort to develop vaccines and antiviral drugs to block the spread of virus and to treat the disease in humans, so as to prevent death.