Influenza viruses are among the most common causes of human respiratory infections [1], and among the most significant because they cause high morbidity and mortality. Influenza outbreaks have apparently occurred since at least the Middle Ages, if not since ancient times [2]. In the elderly, in infants, and in people with chronic diseases, influenza is associated with especially high mortality. In the United States, influenza results in approximately 200,000 hospitalizations and 36,000 deaths in a typical endemic season [3]. In addition to annual winter outbreaks, pandemic influenza viruses occasionally emerge [4,5], as they have every eight to 41 years, for at least several centuries. Up to 50% of the population can be infected in a single pandemic year, and the number of deaths caused by influenza can dramatically exceed what is normally expected [6].