In season 3, Nina Myers is working to acquire the same virus which Jack is trying to acquire. She's working to acquire it for someone else. Is it ever revealed who she was working for?
To my knowledge, this was never revealed.
TV Tropes have it listed as one of their headscratchers (i.e. problems without an answer). There's a great discussion on that link about who the client could have been, but it appears that the answer really is unresolved, with no convincing argument. The actress, Sarah Clarke, did state in an interview that she was exhausted and wanted out of the role, and that the show's creators were appreciate and commandeering of that fact.
That suggests the her role in Day Three was really to be given a proper send off, considering she was a big part of the show.