Hydrogen sulphide
From Tardis Wiki, the free Doctor Who reference
Hydrogen sulphide was a compound made of hydrogen and sulphur. It was produced by black smoker volcanoes. Deep under water, it was a source of energy for the aquatic life.
An alien parasite could only survive in areas with hydrogen sulphide and if it or its host was removed from the source it would die. (BBCR: Submission)
