It's December 1936 when the first polio cases are suspected. Soon a polio epidemic is sweeping the country.
Schools are closed, swimming pools and movie theatres banned to children, and travel restricted - but the epidemic is still spreading.
Tom is the best runner in his school - but you can't outrun polio. Your family can't hide from it. And nobody knows where it will strike next.
One night, I woke up, hot and sweaty, with my heart thumping, and feeling so dizzy that I had to hang onto the edge of the bed. The moonlight was streaming through the window and I could hear Johnny's snorting breaths and feel his toes in my stomach.
Was this how it felt when you were coming down with infantile paralysis? I tried to remember that list of symptoms. Weren't dizziness and fever part of it? Was this how it was going to be? Not Johnny, who was never very strong.
Not Jessie, who was the youngest. The invisible enemy was going to creep in and attack me. I was so scared I couldn’t get back to sleep for ages.
I lay there in the moonlit dark, wiggling my hands and feet and ankles, to make sure that everything still worked properly. Was this it? I would be taken off to hospital. I would never run again, let alone run as fast as Jack Lovelock. I would never get to the Olympics.
I might never even walk again.
Yes, I would. I would make myself get better. Everyone would be talking about me, the boy hero who refused to give up and made a miraculous recovery. I'd be in all the papers. Doctors would come from all around the world to find out how I'd beaten the dread disease.
Or perhaps I wouldn't. I would lie there in hospital for weeks, months, years. All my family would come and visit. Flo and Lily would stand by my bed, sobbing, while the nurses told them how brave I was.
The next thing I knew, it was morning and sunlight was streaming in instead. I tested my fingers and toes again. Everything still worked.
It was all right. I was still safe. We were still all safe.
The story behind the book
The initial idea for Enemy at the gate came from my husband's father, Peter Werry. As a child, he lived with his family in Ashburton, where his father was a teacher. When polio broke out, his parents decided that the safest place for Peter and his brother John would be staying with relatives who had a sheep station at Waikononi, two hours drive away in the Peel Forest.
Even today, the Peel Forest is quite an isolated place. Seventy years ago it must have been even more so. The boys and their mother stayed on the farm, while their father went back to work, but they didn't stay in the farmhouse. They spent three months camping in a tent in a field, not seeing anybody except their relatives and one other family who came with them.
I think the boys quite enjoyed the experience, although it must have been very cold. I don't suppose their mother enjoyed having to sleep in a tent and cook over a primus for three months, but she must have thought it was worth it. In those days, no one knew what caused polio, or how to avoid catching it. There was no cure and parents were terrified that their children would end up sick, paralysed or at worst dead.
• Ask older friends or relatives what they know about polio. If you ask anyone older than about 60, they might remember the polio epidemics. Do people still catch polio anywhere in the world today? Send me an email and tell me what you've found out.
• At the back of the book, you'll find some information about real-life characters in the story. They include royalty, two aviators, a child film star, a Canadian family, a nurse, and even a dog. Which ones are you most interested in? See if you can find out more about them.