How did the antibodies come into contact with the first officer? It's stated the children were in isolation and hadn't come into contact with anyone.
So, how did the antibodies escape isolation and come into contact with the first officer in the first place?
The short answer is that they were only put into isolation when the disease started. Prior to this, they were presumably able to freely mingle with the crew of the USS Lantree, including the officer who had the flu.
KINGSLEY: But the children have been in protective isolation since this was detected. They show no symptoms. You cannot leave them here to die!
Episode Transcript - "TNG: Unnatural Selection"
Interestingly, the line in the original show script was slightly different, albeit it makes the same point about the children only being placed into isolation after the disease took hold:
MANDEL: We have thirty-two children here. They've been kept in positive isolation since the onset, and so far they've shown no symptoms. You must evacuate them as soon as possible.
