The 2009 H1N1 influenza pandemic, caused by a novel virus derived from swine, spread globally, leading to the first influenza pandemic since 1968. The virus primarily affected young adults and children, with a lower impact on older adults. The overall case fatality rate was less than 0.5%, but the risk of severe illness and death was higher among pregnant women, immunosuppressed individuals, and those with underlying medical conditions. Transmission was similar to seasonal influenza, with a basic reproduction number of 1.3 to 1.7. Viral replication was higher in the lungs compared to seasonal influenza, and the virus targeted alveolar lining cells. Early antiviral therapy with oseltamivir was associated with reduced hospitalization and mortality, but resistance to oseltamivir emerged in some cases. The pandemic highlighted the need for continued surveillance, improved diagnostic tools, and better understanding of viral transmission and pathogenesis.The 2009 H1N1 influenza pandemic, caused by a novel virus derived from swine, spread globally, leading to the first influenza pandemic since 1968. The virus primarily affected young adults and children, with a lower impact on older adults. The overall case fatality rate was less than 0.5%, but the risk of severe illness and death was higher among pregnant women, immunosuppressed individuals, and those with underlying medical conditions. Transmission was similar to seasonal influenza, with a basic reproduction number of 1.3 to 1.7. Viral replication was higher in the lungs compared to seasonal influenza, and the virus targeted alveolar lining cells. Early antiviral therapy with oseltamivir was associated with reduced hospitalization and mortality, but resistance to oseltamivir emerged in some cases. The pandemic highlighted the need for continued surveillance, improved diagnostic tools, and better understanding of viral transmission and pathogenesis.