Yellow rain and the natural origin of a “bioweapon”

A recent discussion thread concerning the “lab leak” hypothesis for the origin of SARS-CoV-2 reminded me of another conspiracy theory involving claims of a bioweapon that also probably had a natural origin.

The US government claimed that the Soviet Union was using trichothecene mycotoxins as biological weapons in Southeast Asia in the mid-1970s. In support of this, they pointed to the finding of mycotoxin on leaves and in the blood of people who suffered from seizures, blindness and bleeding, and associated their illness with the appearance of “yellow rain.” A UN investigation was “inconclusive.” No evidence for chemical weapons was discovered, but there was circumstantial evidence for some kind of toxic chemical.

The esteemed Harvard molecular biologist, Dr. Matthew Meselson, and his team went to Laos to investigate these claims. The upshot of their research was that “yellow rain” most likely had a natural origin in well-known mass defecation flights by local bees. The feces were yellow from pollen and were a substrate for fungal growth in the warm, humid jungle.

Does this prove the Soviet Union was not using chemical/biological weapons in Southeast Asia? No. But using Occam’s Razor on the evidence provided by the Meselson investigation, the most parsimonious reading of the available scientific data is that yellow rain had a natural origin.

Likewise, there is no proof that COVID-19 could not possibly have arisen from a lab leak or even an engineered virus. But using Occam’s Razor on the evidence to date, the most parsimonious reading of the available scientific data is that COVID-19 had a natural origin.

Motivated reasoning and confirmation bias are the enemies of good research: believing is seeing. As I tell graduate students, we don’t do experiments “to show,” we do experiments “to test.” Ask questions, but don’t forget to listen for answers, my friends.


A much more detailed history of the yellow rain story is here: https://en.wikipedia.org/wiki/Yellow_rain

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