The US Navy Secretly Sprayed Bacteria Over San Francisco in 1950
In September 1950, a U.S. Navy minesweeper spent six days releasing clouds of Serratia marcescens bacteria just off the coast of San Francisco. Codenamed Operation Sea-Spray, the secret military exercise aimed to measure how easily an enemy could blanket an American coastal city in biological weapons. While military monitors confirmed that nearly all 800,000 residents inhaled the microbes, the experiment led to severe hospital-acquired infections and the death of at least one local patient.
A Cold War Experiment in the Fog
In late September 1950, as the Cold War intensified and American forces were engaged in the Korean War, military planners in Washington were preoccupied with a terrifying scenario: how easily an enemy submarine or cargo vessel could approach the coastline and blanket an entire American metropolis in an invisible cloud of biological agents. To test this vulnerability, the United States military did not build a contained laboratory replica or rely solely on mathematical models. Instead, it decided to test the atmosphere of a real, populated metropolitan area without notifying local officials, healthcare providers, or the public.
From September 20 through September 27, 1950, a United States Navy vessel operated just off the coast of San Francisco, California. Equipped with specialized dissemination equipment, the ship released massive plumes of bacteria into the coastal winds over six days of active testing. The military operation, codenamed Operation Sea-Spray, relied on the natural meteorological patterns of the Bay Area—especially the heavy coastal fog and onshore breezes—to carry the aerosolized particles across the coastline, over the hills, and deep into densely populated neighborhoods.
The operation was designed to establish basic dispersal patterns and dispersion rates for aerosolized biological warfare agents. Military scientists wanted to know whether biological particles would survive the mechanical stress of spraying, how quickly they would dilute as they traveled over land, and whether a single ship could expose an entire urban population before local defense networks could even detect an attack.