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.

SFGate | San Francisco, Biowar test site