The event that caused a global pandemic between 1918 and 1919 was the Spanish flu, caused by the H1N1 influenza virus. This pandemic infected about one-third of the world's population, resulting in an estimated 50 million deaths globally. Its rapid spread was facilitated by troop movements during World War I, creating a perfect storm for transmission. The pandemic significantly impacted public health responses and policies, as governments struggled to manage the crisis. Additionally, it underscored the importance of global health cooperation, which later influenced the establishment of health organizations dedicated to managing future pandemics and improving worldwide health systems.