Day 3: 3:00 P.M.-4:00 P.M. - S3-E3
Plot hole: In the scene when Jack and Nicole isolate the Singers, Jack is informed that they've been tested on site, and the results were negative for the virus, less than 15 minutes after being isolated. However, later in the day the results take several hours to be made available, and have to be processed elsewhere, in a lab. (00:17:45)
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.
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