In 1972, the United States signed "the Convention on the Prohibition of the Development, Production, and Stockpiling of Bacteriological (Biological) and Toxin Weapons and on Their Destruction," along with 108 other countries. But before that, Cold War experiments were carried out in many cities and rural areas across the US to simulate enemy biological warfare attacks, in order to prepare defenses against such attacks. These used common strains of bacteria that they considered benign (Serratia marcescens, Bacillus globigii, Bacillus subtilis, and Aspergillus fumigatus), plus chemicals used as tracers, also thought to be benign, like fluorescent zinc cadmium sulphide. The tests began in 1949 when bacteria was released in the Pentagon's ventilation system. This program moved to US cities in 1950, and over the next 20 years, the US military carried out 239 experiments in 66 cities in the US and Canada.
The public was never informed of these experiments. It was later learned that the bacteria weren't quite benign. Serratia marcescens and Bacillus globigii in particular can cause serious infections. A bacterial spray in San Francisco in 1950 led to eleven people being hospitalized and one man died. It was also later determined that zinc cadmium sulphide is a carcinogen. Read about the biological warfare experiments of the Cold War era and what we learned from them at Today I Found Out.