I dunno, I am mostly in favour of Wormwood just being a horrible disease. Not all that is bright is good (White Court, anyone?), and I can totally understand disease being in Mother Summer's province.
Mind, this is a very straightforward interpretation, so it is a bit surprising attention is paid to it in the book, but maybe it was there to emphasise that while Winter is harsh compared to Summer, it may not be any more or less evil/deadly.