You may be looking for other individuals called Billy.

Billy Davis worked for the Pharm. He was involved in "executive actions". This meant that he killed the clinical trial subjects for the drug Reset before the Mayfly hatched. This made him susceptible, and he breathed in a Mayfly as it searched for a new host. When he was captured by Torchwood Three, the Mayfly within him began to hatch. Owen Harper tried to use the singularity scalpel to save him, but exploded his stomach. Torchwood used his body to get into the Pharm. (TV: Reset)

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