The retro-photonic virus was a disease which made the victim invisible.
While visiting Mohaga, Heather McCrimmon caught the virus. The Tenth Doctor took her to InFECT, where she was examined by Aldrin Strykt. Strykt determined that the virus was draining Heather's energy. The Doctor quickly recovered a cure for the virus from the Mohagan Horde; while he was gone, Heather contaminated the entire staff of InFECT, turning them all invisible. The Doctor returned with the cure and made everyone visible again. (COMIC: The Invisibles)