English: Timeline showing influenza pandemics and epidemics caused by IAVs. The “Spanish Flu” of 1918 was the most devastating influenza pandemic in the 20th century and was likely caused by a zoonotic transmission of an H1N1-type IAV from poultry to humans. This strain disappeared in 1957 when the influenza virus A/H2N2, a reassortant of the H1N1 virus and other avian IAVs, appeared and led to the second influenza pandemic—the “Asian Flu.” In 1968, H3N2, a novel reassortant strain between the H2N2-type and an H3-type virus, displaced the H2N2 strain in the human population and led to the “Hong Kong Flu”—the third influenza pandemic. In 1977 the H1N1 strain reemerged, resulting in the “Russian Flu”. In 2009, a new H1N1 reassortant was transmitted from swine to humans leading to the first pandemic of the 21st century—the “Swine Flu.” In parallel, different avian influenza A virus strains (H5-, H6-, H7-, H9-, and H10-types) have occasionally crossed the host barriers causing mild-to-fatal infections in humans.