Giles Moreau was in charge of Space Dock Nerva. When a virus contaminated members of the crew, he escaped aboard the Chandler with the Fourth Doctor, Leela and Alison Foster.
However, he had already been infected and, in order to avoid contracting the virus, the other three abandoned the vessel. The Doctor had Foster develop an anti-virus so that he could return and cure Moreau. (AUDIO: Destination: Nerva)