So curious how Madeline's mother could possibly have worked as a doctor (exposure to so much illness) and no explanation is offered? Why didn't Madeline ever ask that? Certainly an airlock isn't going to prevent contamination.
In my opinion I think Maddy never asked because it was her mother who was treating her. Maddy had so much trust in her mother that she would never doubt her mother. In the book, they had such a strong and great bond.