The biodata virus was a biological agent capable of editing a victim's biodata, re-writing their timeline, usually with the effect of making them a member of Faction Paradox. It was engineered by Godfather Morlock of Faction Paradox. (PROSE: The Book of the War)
Mathara took the virus to Dust so she could infect I.M. Foreman during their regeneration. However, the virus instead infected the fatally-wounded Third Doctor. (PROSE: Interference)