The poisoning of a Russian spy has ignited a fascination with lethal toxins. You may know about arsenic and cyanide, but these are tame compared with the world's most lethal substances...

Poison, toxin, venom. What are we talking about? Both poison and venom are toxins, because a toxin simply describes a natural chemical that alters the normal function of another organism. There is a quick way to remind you of the difference between poison and venom: If you bite it and you die, it's poison. If it bites you and you die, it's venom.

Toxins are always natural. Poisons can be lab-made.

How was the Russian spy poisoned? The Metropolitan PoliceThe name of London's police force. says the suspected poisoning of Sergei Skripal and his daughter Yulia in Salisbury on Sunday is being treated as attempted murder by a nerve agent. Both remain in a critical condition.

What is a nerve agent? A poison that cannot be produced naturally. One example of a nerve agent is VX, which emerged from research into new insecticides in the early 1950s but proved too toxic to use in farming. VX kills by interfering with the transmission of nerve messages between cells.

It was smeared onto the face of Kim Jong-namKim Jong-nam was the North Korean leader's older half-brother. He was assassinated at Kuala Lumpur International Airport; the four agents thought to be responsible all escaped., the half-brother of North Korean leader Kim Jong-un, at Kuala Lumpur airport last year, killing him in hours.

Which other poisons are commonly used? The most famous is probably arsenic. A naturally occurring element, compoundsA verb meaning to add to something. Or a substance formed when two or more chemical elements are chemically bonded together. In mixtures, the substances present are not chemically bonded together. of arsenic have been used to treat anything from ulcers to syphilis. But a dose of about 100 milligramsThere are 1,000 milligrams in a gram, and there are 1,000 grams in a kilogram. (mg) will show effects within half an hour. It will give you a metallic taste in your mouth, followed by extreme vomiting, seizures and, a couple of hours later, death. It is thought to be what killed Napoleon Bonaparte.

Then there is cyanide, the preferred method of suicide for high-ranking Nazis. If inhaled in high doses it causes cardiac arrest and death within a matter of seconds as it prevents cells from using oxygen to make energy.

But we have not even started talking about the harder stuff...

Such as? Ricin. This toxic plant poison was used to kill the Bulgarian dissident Georgi Markov, exiled in London. In 1978, he was waiting for a bus near Waterloo Bridge, when a man poked him in the leg with the tip of an umbrella laced with a tiny amount of ricin. Markov died three days later.

Then there is polonium-210. This colourless, tasteless substance, known as "the perfect poison", was used to kill former KGBGovernment department of the Soviet Union, which translates as the Committee for State Security. It was responsible for intelligence operations, carrying out a role similar to that of the CIA. agent Alexander LitvinenkoA former spy, Litvinenko fled to Britain after leaving Russia's Federal Security Service (FSB), the successor to the KGB. He then became a fierce critic of the Kremlin. He was assassinated in the UK in 2006. in London in 2006 when it was slipped into his cup of tea.

Aren't some animals poisonous? It takes about 500mg of black mamba venom (two bites) to kill a person. But this is nothing compared to the poison found on the skins of some tiny frogs.

The deadliest of these poisons is called batrachotoxin. Native Indians in Western Colombia collect these frogs and sweat out the poison over a fire before putting it on their darts. An amount the size of two grains of table salt will kill you. One frog has enough poison to kill ten people.

More curious still is the poison found in certain fish, notably pufferfish, called tetrodotoxin. The toxin is the last line of defence for the fish. If a predator manages to swallow them, they release this toxin, killing themselves and the predator. Around 50 people die every year in Japan from eating badly prepared pufferfish.

So what is the deadliest poison known to man? It is called botulinum. The toxin was first identified as a cause of botulismA very rare but life-threatening condition caused by toxins produced by Clostridium botulinum bacteria. The toxins attack the nervous system and cause paralysis. due to incorrectly prepared sausage in late-18th century Germany. Nowadays, unbelievably tiny amounts of botulinum are injected into human faces during Botox treatments.

An injection of just two billionths of a gram directly into the bloodstream is enough to kill an adult. Just two kilograms would be enough to wipe out the entire population of Earth.