Day 7: 11:00 PM - 12:00 AM - S7-E16
Plot hole: The director of CDC informs the cabinet that the biological weapon at hand is a "prion variant indigenous to the East Congo" and refers to it as a "strain". A prion is a protein, not a bacterium, so it cannot be indigenous to a certain place and cannot belong to any strain. More importantly, prion diseases are not known to be transmissible in an airborne fashion (Mad Cow and Kuru, also prion diseases, are transmitted by ingestion), which kinda takes the air out of the threat. However, the description of prion disease symptoms and the example of Creutzfeldt-Jakob disease as being caused by prions are accurate.
Answer: According to Jack in season 5, Nina was "deep cover" before the pair ever met, so it's safe to assume she was recruited before she joined CTU. As for what her primary mission was, who knows? Perhaps it was to gather sensitive intel and pass it on to her superiors. Given that she worked for an intelligence agency, it's not a stretch. Or perhaps we may find out in season seven.
Cubs Fan ★