Here’s what you’ll learn when you read this story:

• During the early decades of the Cold War, the U.S. government conducted 239 open-air germ warfare tests around the country to assess to dangers of a possible chemical attack on civilian populations.
• One of the most infamous, known as Operation Sea-Spray, purposefully pumped aerosols of the bacteria Serratia marcescens and Bacillus globigii—both believed to be harmless to humans—over the San Francisco Bay Area.
• While the operation itself was a success, it’s likely that the test resulted in the death of one man and the sickening of at least 10 others.

San Francisco is known for its fog. Sitting at the intersection of warm air flowing from California’s interior and cool air moving in from the Pacific, low-lying fog and clouds are a common sight. But in 1950, from September 20 until September 27, a different kind of cloud descended on the city of some 800,000 people—a cloud that had been purposefully released by the U.S. government as a secret bioweapons test.

No, this wasn’t some dastardly plan by the government to conduct a macabre experiment on its own citizens. Rather, it was a measure intended to safeguard against other rival nations trying to poison an American city. The government selected San Francisco for its ideal dispersal conditions, tall buildings, and large population, and to pull this off safely, the government relied on the bacteria Serratia marcescens and Bacillus globigii—both believed to be harmless to humans.

“They needed something that was, first of all, thought to be harmless,” Matthew Meselson, a molecular biologist from Harvard, told KQED last year, “because they certainly didn’t want to kill everybody in San Francisco or Oakland. And [they also needed something] that could easily be detected by simple methods.”

Since the Second Battle of Ypres during World War I, when the German army killed thousands of French Algerian colonial troops by unleashing chlorine gas on April 22, 1915, followed by a second gas attack on Canadian troops two days later, nations had been grappling with the threat of unconventional weapons. With its illusion of geographic imperviousness shattered by the attack on Pearl Harbor on December 7, 1941, the United States moved to address its own vulnerabilities. In 1942, President Franklin Roosevelt authorized the creation of the first U.S. biological weapons program. Part of the mission of this program was to determine just how vulnerable U.S. cities could be to a biological attack.

In 1948, the Committee on Biological Warfare—led by University of Wisconsin bacteriologist Ida Balwin—suggested simulating chemical attacks through air, water, and infrastructure (such as subway systems) with non-harmful organisms to understand the threat under real environmental conditions. So, two years later, the stage was set for Operation Sea-Spray, and the entire operation almost went without a hitch. Almost.

Serratia marcescens is found naturally in water and soils, and it’s known to be harmless to humans. But it isn’t typically sprayed in the air in large quantities, and unfortunately, one of those bacteria-filled clouds descended on Stanford University Hospital on Clay Street in San Francisco. There, eleven patients developed inexplicable Serratia marcescens infections. In the case of a 75-year-old Irish American named Edward Nevin, who was recovering from prostate surgery, the bacteria found its way to his heart, killing him. Doctors at the hospital were so puzzled that they even published a scientific paper regarding the infections in October of the following year.

President Richard Nixon ended U.S. research into bioweapons in 1969, and a treasure trove of information about that research was declassified in the 1970s. It revealed that the U.S. had performed 239 open-air germ warfare tests around the country, including in the subway in New York City, on the Pennsylvania turnpike, and in the national airport in Washington D.C. According to KQED, Edward Nevin III—the grandson of the man who died during the faux attack—read these reports and decided to sue the U.S. government, even though he accurately foresaw that he’d eventually lose.

“But we still had to tell the story,” he told KQED. “To have a citizen submitted to that kind of risk is awful.”