S. marcescens was once thought to be a nonpathogenic bacterium. Because of the red pigment it produces, it was widely used to trace bacterial transmission and to study settling and drifting of bacteria in air currents. In 1950, the US Navy conducted a secret experiment called "Operation Seaspray" to study wind currents that might carry biological weapons. They filled balloons with S. marcescens and burst them over San Francisco. Shortly thereafter, doctors in the area noted a drastic increase in pneumonia and urinary tract infections.[6][7]