If Dr Sheila Mackenzie had been awake and conscious she might have been relieved to see she was being taken straight to a hospital and not, as she had feared, to the experimental labs at Porton Down. But inside the specially adapted ambulance she lay in a medically induced coma, seeing and hearing nothing. Her elaborate transfer from plane to ambulance at Brize Norton had had to be explained away to those watching on the base as ‘a snap CBRN exercise’, one of many held periodically around the country to test Britain’s response to a chemical, biological, radiological or nuclear incident. Which was exactly what this was. Since the advent of Covid-19 the sight of medics in all-encompassing PPE gear raised fewer eyebrows than it used to, but still, the same cover story was given now as Dr Mackenzie was rushed through a side door next to A & E, along a corridor, into a lift and finally up into the isolation chamber. Nurses, doctors and several microbiologists stood at a state of alert, wearing full PPE, ready to receive the patient. As the gurney was brought in, with painstaking care, one of them stepped forward.

‘Bring her round,’ he ordered. ‘We need her awake.’

When Sheila Mackenzie opened her eyes, she initially thought she was still in a dream – or, rather, a nightmare. The mask that had been placed on her face in Svalbard had been removed, but she was still restrained on her bed, like a violent prisoner. She could see there were tubes coming out of her arms, an ECG monitor on her chest, and the glassy, unblinking eye of a closed-circuit camera watching her from directly above the bed. She was in an isolation tent, that much she could see, but there appeared to be no one else around. And something else was absent too: the terrible headache that had been building before they loaded her on to the plane in Svalbard and put her to sleep. That at least had subsided. Then a disembodied voice spoke to her and she struggled to tell where it was coming from.

‘Good afternoon, Dr Mackenzie … Please nod your head if you can hear me.’

She nodded.

‘Good. So you will know by now that you have unfortunately been infected with an unidentified pathogen. As a doctor yourself you will appreciate that we urgently need to establish what that is. That’s why in the next few minutes we will need to take live samples from you and expedite them over to Porton Down for diagnostics. In the meantime, until we know its precise structure, we have begun treating you with a broad-spectrum antibiotic.’

Sheila Mackenzie listened to this while a dozen thoughts went through her head. Live samples? This was every bit as bad as she’d feared. What would they show? What were her chances now?