Amelia clutched Ed’s arm with her thin, bony hand. ‘You are our hope, Ed,’ she said. ‘You children. You are not infected. I learnt that much. The young, like us, are not infected.’

‘Why?’ asked Lewis. ‘Why aren’t we infected?’

‘Let’s go back upstairs,’ said Amelia, her voice fragile and fluttery. ‘I’m getting tired. I need to sit down.’

They made their way back to the front room. It took a while for Amelia to get her breath back. She was strong, though. Ed could see that she had to tell them what she knew.

‘You were born after the infection spread across the planet,’ she explained once she was settled. ‘You must think of it like spores. Did you ever see a mushroom pop and spray a little cloud into the air? Spores. Like tiny, tiny little specks of dust. Poof!’

She looked at Kyle, who was not really following any of this, sitting there bored, spinning his axe in his hands and picking dried blood off the blade.

‘You reminded me of it earlier,’ she said.

‘Huh?’ Kyle gave her a vacant, dumb look.

‘When you clapped the arm of your chair.’

‘What have I done now?’

‘Nothing wrong. But do it again.’

‘Do what?’

‘Bang your chair.’ Amelia mimed the action.

Kyle did what he was told – thumped his hand down on to the arm of his chair, sending up a cloud of dust that was picked out by the sunlight coming in through the windows.

‘There,’ said Amelia. ‘You see it? If it didn’t catch the light it would be invisible to you. That’s what happens when a mushroom puffs out its spores. Up they go, into the atmosphere, spread by the wind, and by animals and people. Up into the clouds, down into the lakes and rivers. We think that the disease was spread in a similar way. It must have happened very quickly. And it certainly happened without anyone noticing.

‘The disease would have originated somewhere isolated and there must have been an incident, something that exposed it, and introduced it into the wider world. It broke out of wherever it had been hiding, and it was carried to busier places and incubated unknowingly. And then – pop! – like spores on the wind, it spread itself, and anyone infected became a carrier until – pop! – they passed it on. More spores.’

‘All this happened before we were born?’ Ed asked.

‘It’s the only explanation.’ Amelia looked out of the window, lost in her thoughts for a moment.