Influenza viruses belong to the family Orthomyxoviridae and cause highly contagious respiratory infections in humans and animals. These viruses cause seasonal epidemics and infrequent pandemics in humans. Seasonal influenza epidemics are responsible for between 200,000 and 500,000 influenza-related deaths each year [15] . Avian influenza viruses caused three human pandemics during the last century. The 2009 pandemic, the first pandemic of 21 st century, was caused by a triple reassortant H1N1 influenza virus of swine lineage [16] . In addition to seasonal and pandemic viruses, highly pathogenic avian influenza (HPAI) H5N1 virus has crossed species barrier to infect humans. As of August 9, 2011, more than 500 human cases with over 300 deaths have been reported worldwide [17] . H5N1 viruses replicate to higher titers in lungs and extra-pulmonary tissues leading to acute respiratory distress syndrome, multiple-organ dysfunction, lymphopenia, and hemophagocytosis [18, 19, 20] . Influenza viruses, therefore, pose a constant public health threat, and it is important to understand its pathogenesis to devise effective control measures.