1918 was a year of tragedy, marking both the final year of World War One and the worst infectious disease outbreak in human history. Most are aware that WWI had a devastating impact on our ancestral history, claiming an estimated 16 million lives. The influenza epidemic of 1918, however, infected an estimated 500 million people, nearly a third of the world's population, and caused the deaths of an estimated 50 million people. Yet, it is rarely afforded more than a footnote in the historical accounts of the time.
If you have ancestors who died or disappeared from your family tree between 1918 and 1919, then they may have been victims of the deadly flu pandemic. Do some research into the history of the area your ancestors were living at the time to learn if they may have been affected. Read newspaper reports from your ancestor's communities to learn about the effects of the flu outbreak in the area. Even localities that successfully escaped with few influenza-related deaths left behind an interesting history, as can be seen in the many photographs, articles, and memories documented online in the Influenza Digital Archive from the Center for the History of Medicine at the University of Michigan. Additional firsthand acounts of the influenza epidemic include the Pandemic Influenza Storybook from the U.S. Centers for Disease Control; this interesting account of how Vick's Vaporub was used in staving off Influenza from the Family Oral History blog; and Winding Sheet and a Wooden Box, the first-hand account of Navy nurse Josie Brown.
After you learn the approximate time period when the flu outbreak hit your ancestors' communities, it's worth following up with a search for death certificates, making note of the listed cause of death. Many death certificates from that time period, however, are missing important details such as the cause of death or burial location. This is especially true in areas with a high death toll where many victims may not have been seen by a medical practitioner.