A strange substance fell from the sky over the villages of Laos, Cambodia and Afghanistan in the mid-1970s. It spattered like raindrops, leaving yellow spots after aircraft passed overhead. Communities came to call it “Yellow Rain”.

Soon, reports followed of sudden illness- vomiting, bleeding and blisters- fuelling fears that a new kind of weapon was being used. Survivors carried those stories into refugee camps, where journalists and government officials began recording them.

When the testimonies reached the United States, this ambiguity hardened into a sweeping accusation that the Soviet Union was using biological warfare.