T-2 mycotoxin
T-2 (also known as "Yellow Rain"), a trichothecene mycotoxin, is a naturally-occurring mold byproduct of Fusarium spp fungus which is toxic to humans and animals. The clinical condition it causes is alimentary toxic aleukia and a host of symptoms on organs as diverse as the skin, airway, and stomach.  It is the only mycotoxin known to have been used as a biological weapon, but ingestion may come from moldy whole grains.

# History
T-2 was discovered as a weapon by Russian scientists after a spring harvest delayed by World War II produced flour contaminated with Fusarium and distributed in bread. Many were sickened, some fatally. T-2 has also been suggested as a cause of the Plague of Athens (430 BC). T-2 is an infrequent contaminant in animal feed.
T-2 was proposed as a cause of Gulf War Syndrome for some United States troops exposed to a mortar shell shot by Iraqi forces during the Persian Gulf War and as the substance used to poison Viktor Yushchenko during his 2004 presidential election campaign (though doctors now believe it was dioxin).

## "Yellow Rain" incidents (1975-84)
The name yellow rain derives from a political incident in which the United States attempted to blame the Soviet Union for using chemical warfare agents. In the context of the late Cold War, this charge would have had very serious repercussions, including stalling efforts to get the United States Senate to ratify treaties.
The charges stemmed from events in Laos and Vietnam beginning in 1975, when the two communist governments, which were allied with and supported by the Soviet Union, retaliated against Hmong tribes, peoples who had sided with the United States during the Vietnam War. Refugees fled from what they described as chemical warfare attacks by low-flying aircraft; most of the reports were of a yellow, oily liquid that the Hmong dubbed "yellow rain". Those exposed suffered neurological and physical symptoms including seizures, blindness, and bleeding. Similar reports came from the Vietnamese invasion of Cambodia in 1978.
In 1981, after noting etiological similarities, U.S. chemical weapons experts matched samples taken from an alleged attack in Laos to trichothecene signatures, and hypothesized that T-2 was discovered accidentally through contamination and later weaponized. Secretary of State Alexander Haig announced in September 1981 that
In 1983, a now-declassified CIA report summarized the history of T-2 development in the Soviet Union, where it was referred to as lebeda, Russian for millet or animal feed, and weaponized for aerial delivery.
In 1987, these charges were disputed by Harvard biologist and biological weapons opponent Matthew Meselson and his team, who traveled to Laos and conducted a separate investigation. Meselson's team noted that trichothecene mycotoxins occur naturally in the region and questioned the witness testimony, suggesting an alternate hypothesis that the yellow rain was the harmless fecal matter of honeybees. The Meselson team offered the following as evidence: separate "yellow rain drops" which occurred on the same leaf, and which were "accepted as authentic", consisted largely of pollen; each drop contained a different mix of pollen grains, as one would expect if they came from different bees, and the grains showed properties characteristic of pollen processed by bees (the protein inside the pollen was gone, while the outer indigestible shell remained. Further, the pollen came from plant species typical of the area where the drop was collected.
The different composition is striking, as it implies that the Soviets would have had to have very sophisticated mixing apparatus, that could take pollen from local plants, treat it as done by bees, and then mix the pollen into different drops, such that different drops contain different pollens. In addition, many of the "native people" who made claims that they were attacked by chemical weapons could reasonably be seen as partial to the US.
The question of what exactly "Yellow Rain" was has never been fully resolved.