The Hmong coined the term "yellow rain" in the 1970s, following the communist victories in Southeast Asia. Back then, the new regimes were angry with the Hmong, who had been allies of the U.S. and fought against them. In 1975, the Hmong began reporting they saw an oily, yellow liquid falling from aircraft. The liquid sounded like rain when it hit the roofs of their houses, which led them to dub it "yellow rain."
Soon after, the Hmong suffered a myriad of health issues, including seizures, blindness and bleeding from the nose. Some even died. Yellow rain was also reportedly dropped on the Afghans who fought the Soviet Union's invasion in 1979, and on Khmer tribes in Cambodia around the same time [sources: Schneider, Inglis-Arkell].
So what is yellow rain, exactly? The U.S. government investigated and accused the Soviets of using trichothecene mycotoxins, a poison made from a fungus that can be used as a biological weapon. (The Russians denied their involvement). On the other hand, many scientists, even some American ones, said it could be the feces of giant Asian honeybees swarming through the air in large numbers. Or else, bee poop tainted with a fungus. When the yellow rain dried, it became a dust that contained pollen. Given the geopolitical implications of yellow rain, the source may remain in dispute for a long time [sources: Schneider, Inglis-Arkell].