The 1918 flu pandemic that killed millions.

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.

Flu facts

Influenza, or flu, is a virus that attacks the respiratory system. The flu virus is highly contagious: When an infected person coughs, sneezes or talks, respiratory droplets are generated and transmitted into the air, and can then can be inhaled by anyone nearby. Additionally, a person who touches something with the virus on it and then touches his or her mouth, eyes or nose can become infected.

Flu outbreaks happen every year and vary in severity, depending in part on what type of virus is spreading. (Flu viruses, which are divided into three broad categories, can rapidly mutate.) In the U.S., "flu season" generally runs from late fall into spring. In a typical year, more than 200,000 Americans are hospitalized for flu-related complications, and over the past three decades, there have been some 3,000 to 49,000 flu-related deaths in the U.S. annually, according to the Centers for Disease Control and Prevention. Young children, people over age 65, pregnant women and people with certain medical conditions, such as asthma, diabetes or heart disease, face a higher risk of flu-related complications, including pneumonia, ear and sinus infections and bronchitis. A flu pandemic, such as the one in 1918, occurs when an especially virulent new influenza strain for which there's little or no immunity appears and spreads quickly from person-to-person around the globe.