As the man walked over to her bed, Tan called out her husband's name but got no answer. Then as she reached out to touch the man, 'my hand went right through him', says Tan. All she could think was, 'Lou has died and turned into a ghost'.

Tan, 51, attributed the phantom visitor to extreme fatigue or an overactive dream life. So she and her husband, who had been watching television in the living room, dismissed the matter - until other visions came calling. These included two little girls skipping side by side, a fat poodle dangling from the ceiling, something dead and decaying, plus plenty of aliens and spiders. 'I knew I was hallucinating,' says Tan. 'I just didn't know why.'

Tan was suffering from late-stage Lyme disease, an insidious bacterial illness caused by a deer tick, which has plagued her since 1999. The disease brought on a slew of ailments, including chronic depression, paranoia, agoraphobia, heart palpitations, insomnia, stiff joints and rashes. Yet the cause had escaped detection for four years even though Tan spent at least US$60,000 on doctors, surgery, cat scans and sleep studies. Her brain was so badly affected that she could no longer write.

For someone who has been a fixture on The New York Times bestseller list since 1989 and whose books have been translated into dozens of languages from Estonian to Tagalog, there could have hardly been a more terrible fate.

'The cognitive damage of Lyme is the scariest part of the disease for me,' says Tan. 'I could live with being paralysed, but not with losing my mind?'

Tan's battle against Lyme disease is a long and painful journey that she chronicles in The Opposite Of Fate: A Book Of Musings, (Putnam), her seventh book and her first work of non-fiction. The book is a collection of her speeches, lectures and essays written before she contracted Lyme disease, but Tan has skilfully connected these seemingly disparate pieces by writing an additional 100 pages.

The title derives from what Tan calls her Christian father's sense of faith and her Confucian mother's belief in fate. The book is meant to be an antidote to the weariness and uncertainty with which much of the world has entered the 21st century. Tan contemplates how things happen in her life and beyond, always returning to the question of fate and its opposites: the choices and attitudes that shape our lives.

Tan writes with her special brand of humour, and not a little wisdom. She describes her mother's struggle with Alzheimer's disease with touching wit. Her mother was a rather difficult woman, prone to 'language difficulties, arguments and poor judgments her entire life'.

Although she was diagnosed with Alzheimer's several months before her 80th birthday, the clues to her illness had been evident long before. There was the time in 1991 when Tan was in Beijing with her mother, and the matriarch declined to go into one of the many temples of the Summer Palace. ''Why I go see?' she cried, retreating to a cool stone bench in the shade. 'Soon I just forget I been there anyway,'' writes Tan.

Tan burst on to the literary scene in 1989 with the publication of her first novel, The Joy Luck Club, a partly autobiographical story of four Chinese mothers and their four Chinese-American daughters. The novel remained on The New York Times best-seller list for 40 weeks and in 1994 was made into an acclaimed film. Tan's three other novels - she has written two children's books - have also been bestsellers. This time, she chose to write non-fiction because her thinking was too clouded for crafting a lengthy fictional story.

'I felt what it must be like to have mental retardation,' she says. 'I was grasping for every word. When I went back to read what I had written I didn't recognise it.'

Tan first realised something was badly wrong in 1999, when she had a stiff neck for months. She tried out different pillows but the pain persisted. Then she developed problems with her joints, and could not move her arms.

She suffered from chronic insomnia and her feet went numb in the mornings. She tried sleep tapes, herbs, sleeping pills - all to no avail.

To complicate matters, Tan's mother and literary editor were dying - the former was her muse, the latter her guide in fiction. 'I was under stress and I thought maybe that's what was causing all these symptoms,' says Tan, partly influenced by her physician's conclusion that her troubles were due to middle age.

Says Tan: 'Regarding the muscular stiffness, numb feet and insomnia, I was told 'welcome to the club'.'

In 2000, Tan's fatigue and paranoia intensified. 'I had this overwhelming feeling that somebody was about to attack me,' she recalls. She felt reluctant to go out, going to great lengths to protect herself. She wore a wristwatch that emitted a high-frequency sound audible only to her two Yorkshire terriers, Bubba and Lilli. If she felt uncomfortable around someone, Tan would push a button on the watch, prompting her dogs to bark hysterically. 'I would say, 'Oh sorry, my dogs need to go out', and that would be my excuse to leave.'

In June 2001, Tan embarked on an exhausting four-month book tour across 40 American cities, a dozen in Britain, and to Ireland, Australia and New Zealand to promote her fourth novel, The Bonesetter's Daughter.

'After I came back from the tour I said to Lou, 'I think something is broken in my body. I don't know what it is but I'm not the same person any more'.'

Tan underwent a complete medical check-up, which revealed that her blood sugar levels were as low as those of someone in a coma.

More tests revealed a tumour on her adrenal gland - a growth so tiny and benign that it is called 'incidentaloma'. Tan had the tumour removed anyway, hoping to end her ailments. After the operation, she was put on steroids. Then the hallucinations began.

Steroids, says Tan, suppressed her immune system, allowing the Lyme bacteria in her body to proliferate more freely. 'So while I was feeling better in one sense - my joints were not so achy and inflamed - I was also getting worse.'

Although she has no memory of it now, Tan's behaviour took a bizarre turn. She threw laundry around her New York loft, crammed several boxes' worth of tea bags into a small bowl, and on one occasion pummelled her husband in her sleep.

Worried that she was becoming clinically demented, and possibly even suffering from the early stages of Alzheimer's, Tan and her husband revised their wills and set up a trust. Tan saw a string of doctors but not one offered a conclusive diagnosis, although a neurologist did identify 15 lesions on her brain. Again, Tan wondered if her condition was simply the result of stress and ageing.

Then came an incident that pushed her over the edge. In November last year, Tan was scheduled to appear in Miami with a band called the Rock Bottom Remainders. Its members thought it would be funny if she performed Madonna's Material Girl. Tan spent the flight from San Francisco to Miami trying to commit the lyrics to memory. After six hours, she could not remember the first line. The band has a reputation for ridiculousness, so when Tan fluffed her lines at the performance, the band members laughed. But, says Tan, 'I was mortified.'

Back in San Francisco, Tan made an appointment with another neurologist who tested her for multiple sclerosis, lupus and Lyme. That night, Tan took to the internet with a vengeance. She logged on to the Lyme Disease Association website, where, gasping with horror, she came across the long list of symptoms associated with her illness. Reaching back into her memory, she remembered pulling blood-engorged ticks off her dogs - the animals are immune to Lyme but can be carriers of the disease.

Tan also recalled hiking in many regions of California, where deer ticks and black-legged ticks, both transmitters of Lyme disease, proliferate.

The website suggested that anyone suffering from symptoms of Lyme disease ought to contact a 'Lyme-literate' doctor familiar with the nuances of the illness. Tan found Raphael Stricker in San Francisco, the 11th doctor she would see, and the one to whom she would be deeply grateful 'for restoring my brain to sentence-writing strength'.

When Stricker confirmed that Tan had Lyme disease, she was filled with relief - and revulsion. 'An alien had been in my body for four years, like mice eating away at the wiring of my nervous system,' she says.

Tan was given antibiotics, which initially made her ailment worse and then alleviated some of its symptoms. She now feels energetic and alert but her hands and feet are always half numb. She feels close to 100 per cent mentally, but physically about 70 per cent.

Had her illness gone undetected, she may not have been able to write any more.

Even now Tan will not fully recover until she undergoes three months of intravenous antibiotic treatment, which she plans on doing as soon as she has time. Since her diagnosis, Tan has developed 'a great deal of appreciation for what a healthy body does'. She has also learned to be 'compassionate to anyone with an illness', particularly chronic fatigue syndrome and other problems that do not get taken seriously.

Does Tan feel anger towards the doctors who failed to correctly diagnose her? 'It's not so much anger as disappointment that they were ignorant about Lyme,' she says, adding that most doctors dismiss the disease as hypochondria.

In fact, Lyme is erroneously believed to be a disease found largely on America's east coast, where deer proliferate. Since her diagnosis, Tan has met hundreds of victims, including at least 50 on the US west coast, who have lost their jobs and are turning to poverty programmes to cover their medical costs. Children who remain undiagnosed with Lyme are in a particularly poignant situation.

'They can't concentrate and are labelled as Attention Deficit Disorder cases,' says Tan, who has no children. 'This really affects their confidence and self esteem.'

This month Tan will set up a charity, LymeAid4Kids, for the diagnosis and treatment of children who suffer from Lyme but whose families cannot afford to pay their medical bills. She says she plans to donate to the organisation all the fees she earns during a nationwide lecture tour she is undertaking while promoting her new book. Setting up the charity, says Tan, 'is one good result of my having Lyme disease'.

The other positive thing is that antibiotics have made her skin more sensitive to sunlight, giving her a tan from head to toe. 'People say I look so healthy.'