On September 20, 1950, a US Navy ship just off the coast of San Francisco used a giant hose to spray a cloud of microbes into the air and into the city's famous fog. The military was testing how a biological weapon attack would affect the 800,000 residents of the city.

The people of San Francisco had no idea.

The Navy continued the tests for seven days, potentially causing at least one death. It was one of the first large-scale biological weapon trials that would be conducted under a "germ warfare testing program" that went on for 20 years, from 1949 to 1969. The goal "was to deter [the use of biological weapons] against the United States and its allies and to retaliate if deterrence failed," the government explained later. "Fundamental to the development of a deterrent strategy was the need for a thorough study and analysis of our vulnerability to overt and covert attack."

Of the 239 known tests in that program, San Francisco was notable for two reasons, according to Dr. Leonard Cole, who documented the episode in his book "Clouds of Secrecy: The Army's Germ Warfare Tests Over Populated Areas."

Cole, now the director of the Terror Medicine and Security Program at Rutgers New Jersey Medical School, tells Business Insider that this incident was "notable: first, because it was really early in the program ... but also because of the extraordinary coincidence that took place at Stanford Hospital, beginning days after the Army's tests had taken place."

Hospital staff were so shocked at the appearance of a patient infected with a bacteria, Serratia marcescens, that had never been found in the hospital and was rare in the area, that they published an article about it in a medical journal. The patient, Edward Nevin, died after the infection spread to his heart.

Bacillus subtilis, then known as Bacillus globigii, was one pathogen that was used in testing. WMrapids/Wikimedia Commons

S. marcescens was one of the two types of bacteria the Navy ship had sprayed over the Bay Area.

It wasn't until the 1970s that Americans, as Cole wrote in the book, "learned that for decades they had been serving as experimental animals for agencies of their government."

San Francisco wasn't the first or the last experiment on citizens who hadn't given informed consent.

Other experiments involved testing mind-altering drugs on unsuspecting citizens. In one shocking, well-known incident, government researchers studied the effects of syphilis on black Americans without informing the men that they had the disease — they were told they had "bad blood." Researchers withheld treatment after it became available so they could continue studying the illness, despite the devastating and life-threatening implications of doing so for the men and their families.

But it was the germ warfare tests that Cole focused on.

"All these other tests, while terrible, they affected people counted in the hundreds at most," he says. "But when you talk about exposing millions of people to potential harm, by spreading around certain chemicals or biological agents, the quantitative effect of that is just unbelievable."

"Every one of the [biological and chemical] agents the Army used had been challenged" by medical reports, he says, despite the Army's contention in public hearings that they'd selected "harmless simulants" of biological weapons.

"They're all considered pathogens now," Cole says.

Here are some of the other difficult-to-believe germ warfare experiments that occurred during this dark chapter in US history. These tests were documented in Cole's book and verified by Business Insider using congressional reports and archived news articles.