Archie had said her future would be assured, and she did not doubt his sincerity, but it was the nature of that future that worried her. Assured also meant inescapable. Was a widow and a mother now all she would ever amount to? A life defined and dominated by raising a child was better than one defined and dominated by domestic service, but she had allowed herself to hope for so much more. For all Archie’s talk of wishing her to educate herself and to put that education to practical use, she did not envisage how that could be compatible with the demands of raising a child on her own.

She thought of Mrs Glassford, of the strange and disturbing entity that had consumed her, and could not help but see it as a twisted, grotesque parody of her own condition. She carried inside her a creature that would swell her belly and then devour her, leaching from her the many lives she had dreamed for herself.

The image shook something loose from her head that jolted her back into the present, a terrible possibility forming in her mind.

‘The leeches!’ she said.

‘What leeches?’ Raven asked, understandably confused by her random exclamation.

‘The ones Dr Fowler was bemoaning the loss of. You told me they had fallen off George Porteous and died.’

‘What about them?’

‘Isn’t it strange that they all died at the same time? Have you ever heard of such a thing?’

Raven’s face took on a sudden glow of excitement.

‘Sarah, I believe you have made a crucial observation. It could be that this new malady is a disease of the blood. Perhaps that is in fact how it is carried. As you just said: common ablutions are not enough. If Mary Dempster has been less than scrupulous in washing her hands, tainted blood adhering to them might be the conduit by which the illness is being transferred from one patient to another.’

The excitement in his expression wilted as he failed to see it mirrored in her own. He had leapt, with great energy and enthusiasm, to a predictable but entirely wrong conclusion.

‘Don’t you follow?’ he asked, sounding frustrated. ‘As Simpson found regarding puerperal fever—’

‘Will, permit me to ask you something,’ Sarah said, cutting him off. ‘How many diseases have you observed in gravely ill patients to whom leeches were applied?’

His nose wrinkled in irritation. ‘Dozens. What of it?’

‘And what happened to those leeches?’

‘Nothing. They became engorged and fell off.’

‘After feasting on the blood of patients suffering every variety of disease, the leeches were unaffected. And yet the leeches attached to George Porteous all died. What does that tell you?’

His enthusiasm was rekindling, as though she had merely needed to grasp a principle in order to properly understand.