She had escaped the cholera.

The remoteness of her house; her care never to enter the town; the purity of the water, which trickled always fresh from the cliff close by; and last, but not least, the scrupulous cleanliness which (to do her justice) she had always observed, and in which she had trained up Grace,--all these had kept her safe.
But Grace could see that her dread of the cholera was intense.

She even tried at first to prevent Grace from entering an infected house; but that proposal was answered by a look of horror which shamed her into silence, and she contented herself with all but tabooing Grace; making her change her clothes whenever she came in; refusing to sit with her, almost to eat with her.

But, over and above all this, she had grown moody, peevish, subject to violent bursts of crying, fits of superstitious depression; spent, sometimes, whole days in reading experimental books, arguing with the preachers, gadding to and fro to every sermon, Arminian or Calvinist; and at last even to Church--walking in dry places, poor soul; seeking rest, and finding none.
All this betokened some malady of the mind, rather than of the body; but what that malady was, Grace dare not even try to guess.

Perhaps it was one of the fits of religious melancholy so common in the West country-- like her own, in fact: perhaps it was all "nerves." Her mother was growing old, and had a great deal of business to worry her; and so Grace thrust away the horrible suspicion by little self-deceptions.
She went into the shop.