The emergence of swine-origin H1N1 influenza viruses has raised concerns about the potential for a pandemic. Influenza viruses, which cause annual epidemics and occasional pandemics, have a plastic genome that can undergo mutations and reassortment events, leading to the emergence of new strains with epidemic or pandemic potential. The recent outbreak of swine-origin H1N1 viruses has spread among humans globally, highlighting the need for real-time monitoring and control measures. The mechanisms that determine viral pathogenicity and interspecies transmission are critical for understanding and controlling influenza infections. The Spanish influenza (H1N1) pandemic of 1918-1919, the Asian influenza (H2N2) pandemic of 1957-1958, the Hong Kong influenza (H3N2) pandemic of 1968-1969, and the Russian influenza (H1N1) pandemic of 1977-1978 have all had significant impacts on public health. Highly pathogenic avian H5N1 influenza viruses have also emerged, causing severe respiratory infections and high mortality rates in humans. The recent outbreak of swine-origin H1N1 viruses, which have a similar genetic composition to Eurasian avian-like swine viruses, poses a significant threat. The role of specific viral proteins, such as HA, PB2, NS1, and PB1-F2, in viral pathogenicity and transmission is discussed. The development of improved antiviral drugs and vaccines is crucial for controlling influenza outbreaks and preparing for future pandemics.The emergence of swine-origin H1N1 influenza viruses has raised concerns about the potential for a pandemic. Influenza viruses, which cause annual epidemics and occasional pandemics, have a plastic genome that can undergo mutations and reassortment events, leading to the emergence of new strains with epidemic or pandemic potential. The recent outbreak of swine-origin H1N1 viruses has spread among humans globally, highlighting the need for real-time monitoring and control measures. The mechanisms that determine viral pathogenicity and interspecies transmission are critical for understanding and controlling influenza infections. The Spanish influenza (H1N1) pandemic of 1918-1919, the Asian influenza (H2N2) pandemic of 1957-1958, the Hong Kong influenza (H3N2) pandemic of 1968-1969, and the Russian influenza (H1N1) pandemic of 1977-1978 have all had significant impacts on public health. Highly pathogenic avian H5N1 influenza viruses have also emerged, causing severe respiratory infections and high mortality rates in humans. The recent outbreak of swine-origin H1N1 viruses, which have a similar genetic composition to Eurasian avian-like swine viruses, poses a significant threat. The role of specific viral proteins, such as HA, PB2, NS1, and PB1-F2, in viral pathogenicity and transmission is discussed. The development of improved antiviral drugs and vaccines is crucial for controlling influenza outbreaks and preparing for future pandemics.