Mystery of a Thousand Moons - S1-E18
Continuity mistake: In the previous episode, the Republic troops blasted huge holes into the Separatist lab to enter it. In this episode, in which the highly contagious airborne virus is unleashed into the facility, the holes are conveniently nowhere to be seen, so it can't get loose onto Naboo.
Answer: Dooku never accepted Ventress as his apprentice, as far as I know; he just kept her as his devoted personal assassin. In Phantom Menace, it is stated that there can be only two Sith at a time, to prevent a power struggle in the lower ranks. To accept Ventress as an apprentice, Dooku would have violated that rule.