It sounds like a prime conspiracy theory, and indeed if you type it into Google that’s a lot of what you find, but for a period of at least 20 years, the U.S. army carried out simulated open-air biological warfare attacks – on their own cities.

In the wake of World War II, the United Sates military was suddenly worried about and keen to test out the threats posed by biological warfare. They started experiments looking into how bacteria and their harmful toxins might spread, only using harmless stand-in microbes. They tested these on military bases, infecting soldiers and their families who lived with them, but eventually they stepped things up a notch. Disclosed in 1977, it turns out that the U.S. military carried out 239 secret open-air tests on its own citizens.

In one of its largest experiments – called Operation Sea-Spray – the military used giant hoses to spray a bacterial cloud of Serratia marcescens and Bacillus globigii, both thought to be harmless bacteria at the time, from a Navy ship docked just off the coast of San Francisco. They wanted to investigate how the city's iconic fog might help with the spread of bacterial warfare. And spread it did. It’s estimated that all of the city’s 800,000 residents inhaled millions of the bacteria over the next few weeks as they went about their daily lives none the wiser.

At the time S. marcescens seemed like the ideal proxy for a deadly bacterial attack, like one using anthrax. Living in the soil, it produces a handy, bright, blood-red pigment, a property often exploited in microbiology as a biological marker allowing scientists to track its transmission in various situations. Perfect then, it would seem, to track a simulated biological warfare attack. Except we now know that it’s not the benign bacterium we once thought it was.

The blood red pigment of S. marcescens, useful as a biological marker. Credit: Dbn/Wikimedia Commons, CC BY-SA 3.0.

The military experiments are now known to have caused the death of at least one person, Edward J. Nevin, and the hospitalization of ten others, all of whom suffered from urinary tract infections. It is now known that S. marcescens can cause infection, especially in the urinary and respiratory tract. In fact, it’s even been suggested that the increase in cases of pneumonia in San Francisco following Operation Sea-Spray could also have been a result of the bacterial cloud.

But the experiments didn’t stop there. As stated, the military carried out over 200 such tests across the country, from New York to Washington DC., spraying bacteria and other fluorescent and microscopic particles into the air, one of which – zinc cadmium sulfide – is now thought to cause cancer. In another series of experiments, they even went so far as simulating an attack on Washington’s Greyhound bus station and airport.

And the tests weren’t limited to American shores, either. In conjunction with Ministry of Defence scientists as part of the DICE trials, S. marcescens was also sprayed – along with an anthrax simulant and phenol – from a ship over the coast of Dorset in southern England, and its spread tracked. This is also just one of over a hundred such experiments carried out over the U.K. by the British military, which is known to have sprayed zinc cadmium sulfide across large swathes of the country.