Exactly 100 years ago, on March 11, 1918, a U.S. Army private went to the hospital with cold-like symptoms. He was the first patient diagnosed with what became known as the “Great Influenza,” a pandemic that swept the globe and eventually killed tens of millions of people.

Mysteries about that pandemic endure today. Why did it kill mostly the young and able? Why did the flu hit communities in repeated waves? Epidemiological detectives are still digging through old church records, military archives and cemeteries for clues about exactly what happened and when.

Back in 1918, few people had even heard of viruses, never mind connected one to the deadly pandemic. Tufts University disease ecologist Jonathan Runstadler describes how far the science has come, now recognizing how animals around us can be a reservoir for pathogens that might infect people. Learning more about virus strains in the wild could help scientists predict and respond when an infection spills over to humans and makes people sick.

Influenza still hits every year, and the severity of this flu season surprised health experts. Could a future pandemic also wreak worldwide havoc? University of Florida physician and epidemiologist Nicole Iovine explains that vaccination has minimized the risk of a pandemic, but that “we must continue to support research efforts aimed at developing a universal influenza vaccine.”