Sex Gas
Sex Gas was an intelligent cloud of gas, one of which came to Earth in an meteorite that fell near Cardiff and possessed a local female; Carys Fletcher after being released accidentally by Gwen Cooper. On Earth the gas found the energy produced during sex was the most beneficial (or so it thought). This sex was fatal for the other participant; they (the other participant) were disintegrated and absorbed at the climax.
In its natural form, the sex gas was a thick pink-purple gas with high quantities of vorax and ceranium which could move about at will. It could not survive in the atmosphere of Earth for very long and would quickly become a pink powder.
In order to survive, the gas entered another organism through the mouth and nostrils. Whilst inside a Human or a rat, the sex gas caused a variety of changes, both mental and physical. The host gained an enormous sex drive, sharing in the gas's need for the energy, creating an addiction. The host developed special pheromones which were irresistible to other members of the species of either gender. However whilst attempting to seduce Gwen Cooper it broke off the initiated kiss stating that it need a man to have sex with. Internally, the heart and brain would increase in size and the lungs shrink, which eventually would cause an explosion and kill the host. (TW: Day One)
