On Sunday, a former Russian spy named Sergei Skripal and his daughter collapsed near a bench in Salisbury, England. “Her eyes were just completely white. They were wide-open but just white and [she was] frothing at the mouth,” a man who found the couple told CBS. “Then the man went stiff. His arms stopped moving, but he’s still looking dead straight.”

Classic signs of poisoning with a nerve agent.

British authorities later confirmed it was a nerve agent that sickened Skripal and his daughter, but they declined to name which one. The BBC reports it was not either of the two most infamous nerve agents: sarin, which was used in the 1995 Toyko subway attacks and most likely recently in Syria, or VX, which killed Kim Jong Un’s exiled half brother last year.

But all nerve agents work in similar ways. Molecularly, they are organophosphates. Physically, they are usually colorless liquids. And functionally, they all disable enzymes called acetylcholinesterase, which acts as an off switch in the nervous system.

“You can imagine that if you block one of the major ‘off-switches’ of the body, and are left with all the lights turned ‘on’ all of the time, the body might run into trouble,” a doctor told ABC Science.

All the normal bodily functions go into overdrive: sweating, vomiting, involuntary defecation, frothing at the mouth, convulsions. Eventually, the muscles can give out, leading to paralysis and suffocation. Antidotes are available for nerve agents, but they have to be delivered in time. At high enough doses, nerve agents can kill in minutes.

At lower doses, the symptoms can resemble poisoning with pesticides—and this is not a coincidence. The first nerve agents were accidentally discovered by German scientists trying to develop pesticides. In 1936, a chemist named Gerhard Schrader working for the industrial conglomerate IG Farben synthesized a molecule that worked marvelously at killing insects. The only problem is that it killed apes and other mammals, too. It was not suitable for spraying on fields.

But the information was passed on to the German military, which named the molecule “tabun” after the German word for taboo. Tabun is also known as GA. The G-series nerve agents are so named for their discovery by Germans: sarin (GB), soman (GD), and cyclosarin (GF). Although the Germans stockpiled nerve agents during World War II, they never deployed them. After the war, American and Soviet soldiers came upon the nerve agent stockpiles, and a chemical arms race begun.

Then came even deadlier nerve agents. VX, “venomous agent X,” was discovered in Britain in the 1950s. For years, a similar chemical called VG was sold as the insecticide Amiton, before it was pulled off the market for being too toxic. Since then, more nerve agents have been discovered, including a series of “Novichok” or “newcomer” molecules purportedly developed in the Soviet Union. Nerve agents all have a similar phosphorous backbone, and it is theoretically not difficult to design new ones.

The chemicals can be inhaled, ingested, or swiped on skin or clothes. A police officer who handled the Skripals also fell ill—suggesting whatever poisoned them was through skin or clothing contact.

The police officer is stable now. Skripal and his daughter are also recovering in a British hospital. The nerve agent that poisoned them has likely chemically decomposed. For all their immediate toxicity, nerve agents tend not to last very long.