- You may be looking for Reset (TV story).
Reset was a drug that was being tested by the Pharm on volunteer trial subjects, who were not informed of its true nature.
Reset could cure virtually every known disease by "resetting" the human body back to its "factory setting". However, this was due to the fact that the drug contained the parasitic mayfly in its larval stage, and the larvae need a completely healthy host in which to gestate. These mayflies would effectively hunt down diseased, mutated cells, such as the ones containing incurable or terminal conditions. Normal white blood cells could only attack what they recognised as pathogens, but the mayflies were very adept at destroying what these antibodies failed to destroy, like mutant cancer cells or the sexually transmitted disease HIV. They could even cure diabetes.
The mayflies cannibalised their larval siblings until only the strongest was left. The remaining mayfly then emerged from its host, killing him or her in the process. Should the host die prior to maturation, the surviving larvae had a short time in which they could leave the body and seek out a new host.
To conceal the truth about the drug, the Pharm hired a professional killer to assassinate the trial subjects before the mayflies could emerge, and attempted to erase all medical records of the assassinated victims to further remove any connections to the drug. The Pharm hoped to perfect a safe form of the drug through experimentation and thus earn a Nobel Prize, but never got the chance as their operation was shut down by Torchwood 3. (TV: Reset)