Chemical, not biological, just old-fashioned CW.
From one of the internet's tiny treasures, Delancey Place, February 22, 2022:
Today's selection -- from When We Cease to Understand the World by Benjamín Labatut.
The first gas attack in history happened in Belgium in 1915:
"The horror experienced by the soldiers who survived attacks with sarin, mustard and chlorine gas in the trenches in the First World War had seeped into the subconscious of an entire generation. The greatest testament to the terror caused by history's first weapon of mass destruction was the universal acceptance of the prohibition on gas during the Second World War. The North Americans had enormous reserves ready for deployment, and the British had experimented with anthrax on a remote Scottish island, massacring flocks of sheep and goats. Even Hitler, who showed no qualms when using gas in the extermination camps, refused to do so in fields of war, for although his scientists had manufactured some seven thousand tons of sarin, enough to eradicate the population of thirty cities the size of Paris, he had witnessed its effects first-hand as a foot soldier in the trenches of the First World War, had seen the agony of the dying and had suffered some of its lesser effects himself.
"The first gas attack in history overwhelmed the French troops entrenched near the small town of Ypres, in Belgium. When they awoke on the morning of Thursday, April 22, 1915, the soldiers saw an enormous greenish cloud creeping towards them across no-man's-land. Twice as high as a man and as dense as winter fog, it stretched from one end of the horizon to the other, as far as the eye could see. The leaves withered on the trees as it passed, birds fell dead from the sky; it tinged the pastureland a sickly metallic colour. A scent like pineapple and bleach filled the throats of the soldiers when the gas reacted with the mucus in their lungs, forming hydrochloric acid. As the cloud pooled in the trenches, hundreds of men fell to the ground convulsing, choking on their own phlegm, yellow mucus bubbling in their mouths, their skin turning blue from lack of oxygen.....
....MUCH MORE
