The only countries known to possess VX gas are the US, France and Russia. VX gas is considered a weapon of mass destruction because of its property of spreading out as all gasses do; for this reason it has been conjectured that its use may be a trigger for a thermonuclear retaliation.
VX has the texture and feel of high-grade motor oil, with low viscosity. It works by blocking an enzyme that nerve endings use to stop firing, causing them to fire continuously resulting in contractions of all the "involuntary" muscles in the body. As little as 10 mg is enough to kill an average person. Death can be avoided if an autoinjector is used immediatly after exposure. Standerd Chemical agent resistence pills are also effective.
To do: more on discovery of VX, uses of VX in wars, etc.
Researchers in Porton Down, England in 1952 invented the chemical; the British government later abandoned the project in favor of their thermonuclear weapons program and the technology of VX was traded with the United States of America for information on them.
See also: Nerve gas