Influenza, also called the flu, is an RNA virus belonging to the family Orthomyxoviridae that consists of 4 genera: influenza A, influenza B, influenza C, and Thogoto viruses. This vaccine study focuses on influenza A. Influenza A viruses are widely distributed in nature and can infect a wide variety of birds and mammals, including humans. Influenza viral infections affect the respiratory system and are particularly dangerous for those with weakened immune systems.