Influenza is a respiratory disease caused by influenza A and B viruses in humans. It typically causes annual seasonal epidemics, but sporadic pandemics occur due to zoonotic influenza A strains. The WHO estimates that annual influenza epidemics result in 1 billion infections, 3–5 million severe cases, and 300,000–500,000 deaths. The severity of pandemics depends on viral virulence and pre-existing immunity. The 1918 pandemic caused over 40 million deaths. Influenza vaccines are updated annually to match circulating strains, but their efficacy is limited in cases of antigenic mismatch. Antiviral drugs targeting neuraminidase are used, but their use is limited. Universal influenza vaccines are being developed to provide protection against diverse strains, but their effectiveness needs clinical trials. Influenza viruses are enveloped negative-sense RNA viruses with segmented genomes. Influenza A and B viruses have eight RNA segments encoding viral proteins, including HA and NA, which are the main targets for protective antibodies. Influenza C and D viruses have fewer segments and do not cause severe disease in humans. Influenza A viruses can reassort when infecting the same host, leading to new strains. Avian and swine influenza viruses can cause pandemics, with H1N1 and H3N2 being the main seasonal strains. The 2009 pandemic was caused by a novel H1N1 virus from pigs. Current antiviral drugs have limitations due to resistance, but oseltamivir resistance is not a major public health issue. Seasonal vaccines have suboptimal effectiveness, especially in the elderly, highlighting the need for better understanding of influenza virus biology to develop more effective treatments and vaccines. Influenza A viruses are more likely to cause pandemics than B viruses. Seasonal influenza A H1N1 and H3N2 circulate, with H2N2 being the only human strain from 1957 to 1968. Pandemics occur every 10–50 years, with new strains causing severe disease due to lack of immunity. Influenza viruses spread rapidly during pandemics, with transmission from Asia and the tropics being key sources. Influenza causes 500,000 deaths annually, with the U.S. experiencing 5,000–52,000 deaths per year. Risk factors include age, chronic diseases, pregnancy, and obesity. Influenza can lead to severe pneumonia or secondary bacterial infections. The innate immune system, including interferons, is crucial for viral clearance, but excessive responses can cause immunopathology. Adaptive immune responses, particularly CD8+ T cells, are important for clearing influenza virus. Early CD8+ T cell responses are associated with recovery from severe influenza.Influenza is a respiratory disease caused by influenza A and B viruses in humans. It typically causes annual seasonal epidemics, but sporadic pandemics occur due to zoonotic influenza A strains. The WHO estimates that annual influenza epidemics result in 1 billion infections, 3–5 million severe cases, and 300,000–500,000 deaths. The severity of pandemics depends on viral virulence and pre-existing immunity. The 1918 pandemic caused over 40 million deaths. Influenza vaccines are updated annually to match circulating strains, but their efficacy is limited in cases of antigenic mismatch. Antiviral drugs targeting neuraminidase are used, but their use is limited. Universal influenza vaccines are being developed to provide protection against diverse strains, but their effectiveness needs clinical trials. Influenza viruses are enveloped negative-sense RNA viruses with segmented genomes. Influenza A and B viruses have eight RNA segments encoding viral proteins, including HA and NA, which are the main targets for protective antibodies. Influenza C and D viruses have fewer segments and do not cause severe disease in humans. Influenza A viruses can reassort when infecting the same host, leading to new strains. Avian and swine influenza viruses can cause pandemics, with H1N1 and H3N2 being the main seasonal strains. The 2009 pandemic was caused by a novel H1N1 virus from pigs. Current antiviral drugs have limitations due to resistance, but oseltamivir resistance is not a major public health issue. Seasonal vaccines have suboptimal effectiveness, especially in the elderly, highlighting the need for better understanding of influenza virus biology to develop more effective treatments and vaccines. Influenza A viruses are more likely to cause pandemics than B viruses. Seasonal influenza A H1N1 and H3N2 circulate, with H2N2 being the only human strain from 1957 to 1968. Pandemics occur every 10–50 years, with new strains causing severe disease due to lack of immunity. Influenza viruses spread rapidly during pandemics, with transmission from Asia and the tropics being key sources. Influenza causes 500,000 deaths annually, with the U.S. experiencing 5,000–52,000 deaths per year. Risk factors include age, chronic diseases, pregnancy, and obesity. Influenza can lead to severe pneumonia or secondary bacterial infections. The innate immune system, including interferons, is crucial for viral clearance, but excessive responses can cause immunopathology. Adaptive immune responses, particularly CD8+ T cells, are important for clearing influenza virus. Early CD8+ T cell responses are associated with recovery from severe influenza.