The influenza or flu pandemic of 1918 to 1919, the deadliest in modern history, infected an estimated 500 million people worldwide–about one-third of the planet’s population at the time–and killed an estimated 20 million to 50 million victims. More than 25 percent of the U.S. population became sick, and some 675,000 Americans died during the pandemic. The 1918 flu was first observed in Europe, the U.S. and parts of Asia before swiftly spreading around the world. Surprisingly, many flu victims were young, otherwise healthy adults. At the time, there were no effective drugs or vaccines to treat this killer flu strain or prevent its spread. In the U.S., citizens were ordered to wear masks, and schools, theaters and other public places were shuttered. Researchers later discovered what made the 1918 pandemic so deadly: In many victims, the influenza virus had invaded their lungs and caused pneumonia.