A photograph of Haemophilus influenzae taken through a microscope, 2006. Courtesy of the Center for Disease Control.  

The mystery surrounding the 1918-1919 influenza pandemic has led to many theories about the disease's origin. The prevalent belief at the time was that the disease came from bacteria known as Pfeiffer's bacillus (Bacillus influenza). Others believed the cause was an unbalanced diet or the anxiety of life in wartime. Research since then has led scientists to theorize that the influenza originated as a swine or avian flu, now classified as the Influenza A, H1N1 virus—a disease that remains active today.

There had been prior influenza outbreaks in the US, the most recent pandemic occurring in 1889-1890. However, in previous outbreaks, it was typically the young, the old, and those with compromised immune systems that were the most susceptible. This time, primarily healthy people between the ages of 18 and 49 were afflicted. Symptoms ranged from typical flu symptoms, like aches and pains, fever, dizziness, headache, congestion, and vomiting, to more extreme ones, like hemorrhaging of the eyes and/or ears. Lungs might even fill with fluid, which slowly suffocated the victim. Some victims died within a few days of contracting influenza, whereas others died of secondary complications such as pneumonia.