When people think about poisons, their imagination usually stops at a short list of infamous names. Cyanide, arsenic, and strychnine have dominated stories of murder, espionage, and execution for centuries, earning a reputation as the ultimate chemical killers. Yet from a scientific point of view, these substances are relatively crude. They are dangerous, yes—but they are far from the most lethal compounds known to exist.

Modern toxicology has revealed a much darker hierarchy. Beyond the poisons familiar to history books lies a class of substances so potent that even microscopic quantities can overwhelm the human body. Some occur naturally, produced by plants, animals, or bacteria as evolutionary defenses. Others are the result of deliberate human engineering, designed during periods of intense military research. What they all share is an extraordinary ability to interfere with fundamental biological processes such as nerve signaling, muscle contraction, and cellular metabolism.