My apologies as I am new to this and am not fluent in the meta-language necessary to explain complex grammar, but for me '...as if she had been an infectious disease' doesn't work in terms of what the author is trying to convey. I agree with Apollyon's assessment of the way the sentence comes across.

For me, you need to make a distinction between an imaginary situation in the past from a present perspective (as in 'if I had been an infectious disease...') and an imaginary situation within a narrative structure in which the perspective/point of view is already in the past (as in 'The Lady Blanche avoided her as if she were an infectious disease'). In the former we are looking backwards, requiring past perfect, in the latter we are looking 'across', requiring past simple.

There is probably a much better way of putting this!