Local health departments warned those who were ill to stay away from theaters and other public places. This poster was displayed by the Chicago Health Department in theaters providing crucial information influenza pandemic. The 1918 flu pandemic (Spanish flu) was an influenza pandemic. It was an unusually severe and deadly pandemic that spread across the world. Most victims were healthy young adults, in contrast to most influenza outbreaks, which predominantly affect juvenile, elderly, or weakened patients. The pandemic lasted from June 1918 to December 1920. Between 50 and 100 million died, making it one of the deadliest natural disasters in human history.