The biodata virus was a virus that altered the biodata of its victim so that they became a member of Faction Paradox. It was created by Morlock. (PROSE: The Book of the War) It could only infect a Gallifreyan if they were regenerating.
Mathara took the virus to Dust so she could infect I.M. Foreman during their regeneration. All did not go to plan, and the virus instead infected the Third Doctor after he was fatally wounded. (PROSE: Interference - Book Two)