... Or it sounds like what we are calling infection really isn't in the classic sense of the word ...
I
very much doubt that Nemesis works identically to
any mundane biological infectious disease!
It's merely a good simile in some specific narrow-focus cases.
... Since Mab seemed to be blaming it as a source for Maeve's infection, it implies that she had contact with it also ...
I don't think that's implied, at all. "
It was the knife" is simply a statement of
how Nemfection entered the Winter Court.
Whether Lea then
used the Athame to nemfect Maeve, or whether
Lea herself was the Nemvector that got Maeve (or whether Lea nemfected some unrevealed additional agent (unseen onstage) who in turn got to Maeve) isn't -- AFAIK -- addressed by Mab's statement.
Another possibilty: what if it was
Mab herself who, in forcing Nemesis out of Lea (there at the Winter Well), allowed it (unbeknownst to Mab) to enter Maeve? Extra pain and rage for Mab!
But I think Cat Sith is yet another instance, unrelated to the Lea/Maeve nemfection.