The Army kept the biological-warfare tests secret until word of them was leaked to the press in the 1970s. Between 1949 and 1969, when President Nixon ordered the Pentagon's biological weapons destroyed, open-air tests of biological agents were conducted 239 times, according to the Army's testimony in 1977 before the Senate's subcommittee on health. In 80 of those experiments, the Army said it used live bacteria that its researchers at the time thought were harmless, such as the Serratia that was showered on San Francisco. In the others, it used inert chemicals to simulate bacteria.

"...the Army had staged a mock biological attack on San Francisco, secretly spraying the city with Serratia and other agents thought to be harmless. The goal: to see what might happen in a real germ-warfare attack."

This is a scary thing to think about. Why they hadn't discovered that Serratia was harmful yet is something I am going to research.