Sex Gas is a type of intelligent cloud of gas, one of which came to Earth in an meteorite which fell near Cardiff and possessed a local girl named Carys Fletcher after being released accidentally by Gwen Cooper. The gas on Earth found the energy produced during sex was the most benifical (or so it thought). This sex is fatal for the other participant, since they were disintegrated and absorbed at the climax.
In its natural form, the sex gas is a thick purple gas with high quantities of vorax and ceranium which can move about at will. It cannot survive in the atmosphere of Earth for very long and will quickly turn into a pink powder.
In order to survive, the gas enters another organism through the mouth and nostrils. While inside a Human or a rat, the sex gas causes a variety of changes, both mental and physical. The host gains an enormous sex drive, sharing in the gas's addiction. The host will develop special pheromones which are irresistible for other members of the species of either gender. Although Carys Fletcher's pheromones affected Gwen Cooper, Fletcher broke off their kiss and said that the entity inside needed to have sex with a man. Internally, the heart and brain will increase in size and the lungs will shrink, which will eventually cause an explosion and kill the host. (TW: Day One)