Agent Yellow

Agent Yellow was a virus that turned oxygen into sulphuric acid. According to one account, Liz Shaw died from this virus in 2003. The Seventh Doctor was able to devise an antidote known as Agent Scarlet. (PROSE: Eternity Weeps) However, other accounts posited her still being alive and at least informally attached to UNIT in 2010, years after she was reported to have died by this virus. (TV: Death of the Doctor)