The Mayfly was an insect used by medical research facility The Pharm, run by Professor Aaron Copley as part of a drug that was capable of resetting a human body suffering from normally fatal diseases to a healthy state. However, it would eventually kill the host as the mayfly larva grew inside the body. During Torchwood's investigation of The Pharm, Martha Jones was briefly infested with a mayfly, and was the only person to survive because her travels in the TARDIS had improved her immune system. Owen was able to teleport the mayfly out of her body before it could injure her. After Ianto and Gwen discovered the parent mayflies behind the Pharm, Jack had the entire facility destroyed so that the mayflies and other aliens could no longer be used for experiments.