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. Victims died within hours or days of their symptoms appearing, their skin turning blue and their lungs filling with fluid that caused them to suffocate. In just one year, 1918, the average life expectancy in America plummeted by a dozen years.

"Children's Nursery Rhyme"

There was a little girl, and she had a little bird,
And she called it by the pretty name of Enza;
But one day it flew away, but it didn't go to stay,
For when she raised the window, in-flu-Enza.

Today, 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.

D.A.B, Wash, Rinse, Repeat
After Destroying. All. Bacteria. "The number one thing you can do to protect yourself from a cold or flu is to wash your hands thoroughly and frequently," says microbiologist Andrew Pekosz, Ph.D., of Johns Hopkins University. Lather up with running water and soap (antibacterial soaps are no better than the regular stuff) and scrub your palms, between your fingers, and the backs of your hands for a minimum of 20 seconds. Or, in a pinch, you can use hand sanitizer that's at least 60 percent alcohol, according to the Centers for Disease Control and Prevention.

Sources: History.com & PBS.Com