Letter carrier in New York wearing mask for protection against influenza. New York City, October 16, 1918. Letter carriers, mass transit workers, and others who came in contact with the public, were especially vulnerable to disease. Wearing a face mask helped them avoid contagion. The 1918 flu pandemic was an influenza pandemic. It was an unusually deadly and severe pandemic that spread across the world. Historical and epidemiological data are inadequate to identify the geographic origin. Most victims were healthy young adults, in contrast to most influenza outbreaks, which predominantly affect juvenile, elderly, or weakened patients.