Blood agent
The first blood agent to be used as a weapon of mass destruction was phosgene.  Phosgene was first used by the German Army to attack the British Army in 1915.  After that, phosgene was used often during World War I.  Countries on both sides of the war - including Germany, France, the United Kingdom, and the United States - used phosgene to attack enemy soldiers.  Often they mixed it with chlorine, with the goal of killing more people.  Phosgene caused about 85% of the 100,000 deaths caused by poison gas during the war (a total of about 85,000 deaths).