I send her a picture of Kleopatra, an asteroid shaped like a dog’s bone. A few minutes later she replies: OMG! We have so much in common!!!

That’s when I remember Elsa. This time when I rest my hand on her forehead it feels far hotter than before and her cheeks are bright red. I take her temperature and the thermometer reads thirty-nine degrees.

That can’t be good. I check a few websites and they all seem to agree that you need to contact a health professional if a baby’s temperature goes over forty, but I can’t do that. What would a doctor say if they came round here and found us on our own? Before, when Elsa’s been ill, Mum’s taken her to the out-of-hours surgery at the hospital, but a fifteen-year-old can’t turn up with her baby sister with some lame story about her mum’s slipped disc.

I try Grandad’s mobile but it goes straight to voicemail, and there’s no answer at his flat. He said he’d be back ‘in an hour or so’, but he’s already been gone for over two. I stare down at Elsa as I try to decide what to do. She’s started making these whimpery noises like she’s halfway between being asleep and awake. I put the thermometer on her forehead and this time it reads forty. I can’t wait for Grandad – who knows what’s distracting him. I’m just going to have to get Elsa’s temperature down myself.

I lift Elsa out of her cot and lie her on Mum’s bed. Her head flops to one side, but she still doesn’t wake up. I take her out of her sleep bag and pull open the poppers on her Babygro. Without touching her, I can feel the heat radiating off her chest.

‘Elsa?’ I give her shoulder a shake. ‘Wake up. I’ve got some yummy medicine for you.’

Her eyes flutter open, but when she sees me, her face crumples and she starts crying. It’s not her usual scream, but something much weaker. I try to squeeze the medicine into her mouth, but she shakes her head and half of it ends up dribbling down her chin.

‘Now you’re going to be sticky and hot,’ I say, trying to hide the panic that’s building up inside me. I get her to drink some water from a sucky mug, but then she pushes it away, and carries on crying.

‘Elsa.’ I lie next to her and hold her foot. I don’t want to get any closer in case I make her even hotter. ‘I know you feel horrible but it’s really a brilliant thing that you feel this ill.’ Still whimpering, she sticks her thumb in her mouth. ‘Really!’ I say. ‘You have an infection and your amazing body is fighting it. Right now, your inner core,’ I draw a circle on her chest, ‘is getting hotter. Your body is killing the bacteria and viruses inside you.’ She grabs hold of my finger. ‘See, you’re already getting stronger!’