Alice LaPlante hadn't written a novel before. She doesn't read crime or mystery books. And yet her debut novel, the mystery Turn of Mind, has just become the first work of fiction ever to win the Wellcome Trust prize for books on the theme of medicine, beating the likes of Philip Roth, Ann Patchett and Siddhartha Mukherjee's epic "biography" of cancer to the £25,000 pot.

It's an unlikely success story, but perhaps even more unlikely is the premise around which she built Turn of Mind. It's a murder mystery, told from the perspective of a suspect with Alzheimer's. LaPlante (no relation to Lynda) has taught creative writing in the US for 20 years and was so interested in the notion of a suspect who couldn't remember her actions that she even composed a textbook, Method and Madness, on the subject. An earlier attempt at a novel didn't work and she finally "put it to death" about a month before she began writing Turn of Mind. The problem with her first attempt, she says, was that it didn't come from an "urgent place". Turn of Mind, which drew from her own experience of her mother's Alzheimer's, did; the text just flowed out of her.

"I live in California but I go back to Chicago where my family is as often as I can," says the quietly spoken academic, on the phone from the States. "I'd just come back from a very brutal trip home, it had been particularly difficult, and I was kind of collapsed on the couch. We were watching Sherlock Holmes, and my husband said 'why don't you forget all this literary writing, just write something fun, write a mystery'. I thought 'I can't write a mystery, I don't read mysteries.' A few minutes later I said 'wouldn't it be a funny, funny haha idea, given we'd just been talking about how awful Alzheimer's is, if you had a detective who had Alzheimer's, who couldn't remember the clues and was trying to solve a case?' He said 'why don't you write that?' I said I couldn't, I didn't know how a detective would think. But then I thought - you know, I could do it from the point of view of the suspect. And I sat down that night and wrote the first section and it really didn't change much after that."

She'd tried approaching the subject of Alzheimer's from different perspectives in the past - through her diary, as a short story – but "it's a hard thing to frame". "It brings up very intense emotions. I tried non-fiction, I tried journalling, I tried a short story, but I could never find a way into the material without it being really dramatic or sentimental."

Then came the mystery idea. "To me the mystery was just a frame. Just a structure," she says. "It got me into writing it and allowed me to write about it in a way that allowed me to access my material. It gives readers a way in as well. I think without it if you tried writing a piece of traditional literary fiction about an Alzheimer's deterioration, I don't know how many people would read it. It gave me something to hang everything off of."

Told from the perspective of Jennifer White, a brilliant mind spiralling into the abyss of Alzheimer's, Turn of Mind circles obsessively around the death of Jennifer's best friend Amanda. Amanda has been found murdered, with four fingers removed from her hand; Jennifer, a brilliant hand surgeon, is a suspect, but she cannot even remember that her friend is dead, let alone whether she killed her.

Disorientating, told in fragments, the novel witnesses the disintegration of Jennifer's mind as the dark secrets of her family slowly come to light. Judges for the Wellcome prize called it "technically daring", and said that it "emphatically confirms the ability of literature to tell us more about the heart and soul of an illness than any textbook".

For LaPlante, diving into the heart and soul of Alzheimer's was, she says, cathartic. "My mother is in her last stages right now. It's more peaceful because she's weaker but she seems to have been suffering a particularly nasty strain. Her particular experience has been very, very difficult, very hard on her and of course on everyone around her," she says. "I wanted to put myself in that position, to understand what my mother's going through, and I just imagined myself into it. I loved it. I loved every minute of writing that book."

Making Jennifer so strong, so analytical, so – on occasion – humorous about her disease, was, she thinks, "a way of psychologically protecting" herself. "She did feel pain, she felt tremendous pain, but she was strong enough to cope with it. I think that was my way of comforting myself as I wrote it, to put her in these situations and have her deal. Most of the pain I would say is felt by the people around her. She really copes quite splendidly with it," says LaPlante. "My mother has not been able to deal very well with it. It's just heartbreaking. Heartbreaking."

Although the characters in Turn of Mind are in no way autobiographical, before she finished the novel LaPlante called her father, to ask him if he would feel comfortable with it. "I didn't want him to think I was exploiting our family pain. But he was wonderful about it, he said 'absolutely no problem'."

Despite this, he has been unable to read the finished novel, finding it too distressing. "I get that, that's fine," says LaPlante. "What he was responding to is there's emotional stuff he recognised, even if none of the actual facts are the same. The emotional things that go on between people in those situations, he did recognise those and they were drawn from my life, and he found that very difficult to read."

The author is hopeful that Turn of Mind will help start conversations about Alzheimer's. "That would be wonderful. Over here there is more and more chatter about Alzheimer's but it's more stated as fear; as 'I hope I don't get that when I get old'," she says. "But the statistics, as they are in the UK, are very alarming, about the sheer numbers we will have because of the ageing population. [And] we are so unprepared. We simply do not have the infrastructure, the healthcare infrastructure, the financial infrastructure, the emotional infrastructure for families to deal with this. We are so unprepared on every level. And I think it's one of those things people aren't going to realise until they're hit with it: this is going to wipe out your savings, and your children's savings. And one of your kids is going to have to quit their jobs to take care of you. It just devastates families."

There are, believes LaPlante, "going to be so many people" who get Alzheimer's – and she fully expects she will be one of them. "We've always joked that we're at ground zero for Alzheimer's. Our family is just riddled with it from way back. That's probably why my mother was diagnosed fairly quickly and fairly early. Her mother, her grandmother, her sister, her cousin, all died of it, and so we had our eyes on her, and she had her eyes on herself. I just assume I'm going to get it. I'm planning for that. I'm planning financially, I'm planning emotionally, I just assume that's my future," she says. "I'm 53 so I have about 20 years. Let's keep our fingers crossed. You cannot deal with this disease by being in denial and that's what's tearing families apart here. I see it all the time."

For now, she's "almost done" with her second novel, the story of a 17-year-old girl who becomes obsessed with a doomsday cult. "It's quite different," she says. "It's a comedy that ends happily."