Drawing of the 1918 influenza in lungs: Main bronchus of a set of influenza lungs, showing congestion of vessels and infiltration of round cells, including thickening of the membrane. John George Adam, 1920-21. The 1918 flu pandemic (lasting from January 1918 to December 1920) was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus. It infected 500 million people across the world, including remote Pacific islands and the Arctic, and killed 50 to 100 million of them - three to five percent of the world's population - making it one of the deadliest natural disasters in human history.