"So… Spinning heads?"

Sister Simmons rolled her eyes at me. I’d never liked her much, but she had deep roots in the community, and knew things I didn’t.

"Spinning heads," she sighed. "Not likely to improve the host's health, is it?"

She was shorter than me, but I had to hurry to keep up. She walked like she was in constant need of the loo.

"I'm just starting with what I know about exorcism," I said. "Which is nothing."

"Less than nothing," she corrected. "What you know is what Hollywood has taught you. Let your first lesson be to unlearn all of it."

I prayed for patience. Found a little.

"I’ll do my best," I said carefully. "I'd just like to know what to expect."

"You can expect nothing so dramatic," she said, "As spinning heads or eighth-grade vulgarity." Her tone was as clipped as a formal hedge. "Consider the demon's objectives. First and foremost is the health of its host-body. Second, to remain undetected for as long as possible. And third, to subvert as many potential hosts as it can."

"So, it's starting a multi-level marketing company?"

The look she gave me was bespectacled cyanide.

"You’re closer to the mark than you realise," she said. "The demon will indeed focus on its host's closest contacts, those it can exert the most influence upon. Demons spread, always, along social vectors. Here we are."

We’d stopped by a disappointingly ordinary house. I'd expected something grand, perhaps a little gothic.

"Well, up you go!" ordered Simmons. "You're the priest, I'm merely a nun. They'll want you to see you first."

Swallowing a wholly impious retort, I hurried up the concrete steps and knocked on the peeling burgundy door. Straight away I heard padding footsteps, and the door swung open.

A man stood there in greying pyjamas, balding and rather pasty.

"Yes?" His voice was high, agreeable.

"Mr Ivan?" I asked.

"Yes?"

"My name is Father Poldridge," I said with a hand to my chest. "And this is Sister Simmons. May we come in for a chat?"

The man looked at my hand on my chest, then the collar at my neck, and then in my eyes.

I felt a chill. There was something about the way his eyes moved – too slow, too smooth. Like in a video game.

"Yes," came a clipped voice from behind me. "He never could get the eyes right. All those fast-twitch muscle fibres, I expect."

Before I could turn, something pushed me in the middle of my back with such force that I flew through the doorway. I skidded on polished boards before thudding into a wall, winded. Gasping for breath, I twisted my head to face the only source of light. Sister Simmons stood silhouetted in the doorway, Mr Ivan cringing in her shadow.

"I did tell you," said Sister Simmons, stepping over the threshold, "Demons spread along social vectors.” Her smile flashed too wide, too white. “And yours is such a popular parish."