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Influenza A and B viruses are considerable public health threats. Annual seasonal epidemics are driven by antigenic drift and cause an estimated 3-5 million severe cases and 290,000-650,000 deaths annually. The clinical presentation of seasonal influenza is typically an abrupt, uncomplicated acute respiratory illness with full recovery; however, it can lead to severe, life-threatening complications in individuals at high risk. Antigenic shift caused by reassortment of a seasonal influenza A virus with avian and/or swine influenza A viruses has led to unpredictable pandemics. The successful cross-species transmission of influenza A viruses requires key viral adaptations, including changes in receptor specificity and compatibility with host factors such as ANP32A. Host defence involves a layered innate response, which is antagonized by proteins, such as non-structural protein 1, and a robust adaptive immunity comprising cytotoxic CD8⁺ T cells targeting conserved internal proteins and B cells producing neutralizing antibodies. Diagnosis primarily depends on highly sensitive PCR-based methods and multiplexed rapid antigen tests. Prevention involves annually updated seasonal vaccines (including inactivated, live attenuated and protein-based vaccines), while treatment relies on early use of neuraminidase and polymerase acidic protein inhibitors. Research priorities include the development of improved vaccines and a better understanding of influenza virus transmission from animals to humans.