For some people, the instinct in times of serious illness is to close the shutters, switch on the answering machine and endure whatever lies ahead in the privacy of a tight circle of family and close friends. But for others, sickness feels more bearable when it is a public affair - when the one who is ill draws in a wide circle of friends and acquaintances ... or, in some cases, the world, through the medium of personal health weblogs.

The written word is one of the means human beings use to purge emotions, heal or comfort each other, writer and reader alike. When BBC online writer Ivan Noble died late last week, 300,000 people logged on to read his final post on the web diary that he had begun to chart his experiences after being diagnosed with a malignant brain tumor in August 2002.

During the life of the diary, fellow bloggers across the world wide web offered countless words of comment and comfort on Noble's site and on websites of their own - all examples of writers and readers forming an online community of suffering and support in the field of health.

Every voice is different, but the basic role for most of these bloggers entails posting regular personal entries, responding to readers' emails and comments, and keeping the site updated with pictures, archives and links to other web pages that the blogger finds interesting. All this becomes a job in itself, and in some cases an added incentive to live.

One blogger wrote of finding that his web diary helped him just as much as it offered advice and inspiration to others: "I know that it has kept me going much longer than I would have without it, and I am grateful."

Many health blogs reveal how the bloggers' husbands, wives or friends remind and encourage them to post their thoughts as often as possible, as if this wall of words helps to keep the sickness at bay.

And when it comes to generalised outbreaks, such as the Sars epidemic of 2003: "The thing about blogs is that they are a good way of dealing with public anxiety about emerging diseases," says Trevor Jackson of the British Medical Journal.

Over the past months, DaveyBoy, an HIV-positive blogger from Toronto, Canada, has written many tense entries for HIV-Aids.blogspot.com as he waits to find out whether his newborn baby has contracted HIV from his mother. In October, 10 days after Ethan was born, he wrote: "Still worried about Ethan's HIV status, still worry he got HIV from mom or during delivery. Even if he isn't HIV-positive, I still will have to be careful the rest of my life around him as my blood is infected." Three tests later, and things are looking good. Ethan has tested negative for them all, and has just one more to confirm his clear status.

Laura, who was 58 when she started her weblog in July last year, is going through the menopause. On Is It Hot in Here?, she documents her search for natural and safe HRT options, charting the highs and the lows of each hormone treatment. "No hot flushes and no sex drive! That's the good news and the bad news for the new year," she wrote on January 19.

Dr Sarah Rawlings, policy manager at the charity Breakthrough Breast Cancer, says: "We know from talking to women with breast cancer that, for many, talking or writing about their experiences can help them cope with their diagnosis and subsequent treatment. Online diaries, such as blogs, are one way of doing this, and often provide reassurance for other patients. They can also help family and friends understand what a loved one is going through at a particularly difficult time."

But could it be that our hunger for weblogs that trail the traumas of the sick is mainly a morbid fascination with bodily functions, death and disease? Emma Candy, who was diagnosed with ovarian cancer in June 2003 when she was 35 years old, wrote a blog charting her treatment, excerpts of which were published in the Guardian after she died in February last year.

Like many health bloggers, she often reflects on the meaning of her diary posts and the psychology behind them. "Not sure what my blog is meant to be any more," she writes on November 12. "Is it a daily diary? A photo-love story? A eulogy to family and friends? A resource for others in the same situation?"

In the tradition of theatrical tragicomedies, however, Emma seems to cope with the most difficult situations through humour, a feature common to many health bloggers. In the darkest moments, a light touch can offer relief or perspective. And laughter unites people, bringing about the comfort of company.

Cass Brown, author of the popular weblog Cancer Giggles, also has humour close to his heart, describing his website as "an idiot's guide to accepting, living with, laughing at and dying from cancer".

And Dori Johnson - who was first diagnosed with breast cancer in 1998 - imbues her posts with irony and wit. Archived pictures of her hair loss, showing her when blonde and when bald, are called "Hair today and ... gone tomorrow". And one day in November last year, under the heading Simplifying Life, she writes: "Went to trim my toenail today and the whole thing just came off! Sure makes that job easy. Such are the joys of chemotherapy ... Hope nothing serious falls off. Happy new year to everyone."

Dori also puts her cancer into perspective by blogging about the other things in her world - her favourite recipes, for example, and her journey towards adopting a child after she found she could have no more of her own.

The following is taken from No Bra Required, posted by Dori's husband, JE, in December 2002. "Dori will probably not like me sharing this, but sometimes you just got to get things off your chest. Last night we played a huge round of "what if" with all the big questions we are facing right now. For example, what if Dori's disability claim is denied? if can't sell the Yukon; if we can't pay the property taxes in January; if I lost my job. All things out of our control and not worth even worrying about. But Dori worries anyway and makes an already stressful time of year and time in our lives even more so. In the end we got it all out there and I think we both feel better. Change is certainly coming and we will do what we need to do. Everything will be all right. We decided that, no matter what, as long as we had each other and our kids and family, we would be very happy together."

One of Dori's recommended weblogs, I Will Survive, supports this view. Its author, Sandee, was diagnosed with breast cancer in 1998. The cancer, which she calls the Dragon, has now spread to her bones. Sandee says her website is a place to escape, somewhere that keeps her mind off her illness - even though, paradoxically, cancer is the weblog's driving force.

"I've been having more pain in my spine than usual this week, this puts me in worry mode," she writes. "Blows my mind that there is no cure for bone metastasis. They say that the average survival after the diagnosis of a breast cancer metastasis to bone has dramatically improved to about 24-36 months. Sigh ... is this supposed to make me feel better?"

Technologically, the blog is a phenomenon of the web age - but the union of health and the diarist has a long tradition. John Mullan, senior lecturer in English at University College London, notes the similarities between 18th-century journals and today's weblogs in their unflinching honesty about personal health. From the religious confessionals of Medieval English literature to the modern diaries of Anne Frank, Virginia Woolf and Sylvia Plath, "One of the things that diary keepers did was monitor their health," he says. "It wasn't for anybody else, it was part of what self-inspection was about. They told you what they ate and what stomach upsets they had. They bothered with all the day-to-day trivia."

In 1812, for example, Fanny Burney wrote a journal about her mastectomy without anaesthetic. "It's an extraordinary account of her symptoms, how she was diagnosed with cancer," says Mullan. "And then, in excruciating detail, the operation itself." Health weblogs do the same, with cancer sufferers recording intimate details of their chemo- therapy, medication and surgery. The reader is spared no detail. Fanny Burney even got her husband to comment on the operation, which is again reflected in the habit of bloggers who include comments from their friends and family, especially when they are too sick to post entries themselves.

Back in our own time, thousands of readers have found good in the web words of the likes of Ivan Noble. As his friend, Simon Fraser, wrote: "Some wanted to share cancer experiences and took strength from Ivan's courage and openness; others wanted to offer him support."

Noble lives on in the minds and words of many bloggers due to a request in his final post: "I will end with a plea. I still have no idea why I ended up with a cancer, but plenty of other cancer patients know what made them ill. If two or three people stop smoking as a result of anything I have ever written, then the one of them who would have got cancer will live, and all my scribblings will have been worthwhile."

Many have taken his words to heart. One blogger writes, "If I hadn't given up smoking last year, I'd be stopping today." Another: "After reading his last few sentences, I made a promise to myself to give up the ciggies, not even allowing myself the odd one. I will think of him every time I'm tempted, and remember that when we're given a chance to prevent cancer, we should take it."

Re-reading their posts brings these health bloggers to life in such a forceful way that you catch your breath when you remember that some of them have died. The simple tales of living day to day with a life-threatening disease are unimaginably touching and can help families and friends of those suffering understand the experience.

Even for those who have chosen not to share their stories on the internet, Ivan Noble's diary has been very evocative. "My wife died 18 months ago from cancer and I can only think now what [Noble's] family are going through," says one correspondent on his website. "I did think about blogging my experiences of widowerhood, which now would have helped people like [Noble's] wife, but coming to terms with grief and keeping a diary about it was too much for me."

And as another reader, Iain, wrote of Noble: "Ivan, you started writing this column at about the time I was diagnosed with cancer. Thankfully, I have come through it. I owe a lot to you - it was you who gave me perspective, hope and determination. I made my family members read it to help them understand. I still recommend your column to people I know who fall ill. I will miss your writing."

· Ivan Noble's Tumour Diary is at news.bbc.co.uk/1/hi/health/4211475.stm
May I Be Frank?: www.mayibefrank.typepad.com
Is It Hot in Here?: www.healthdiaries.com/blogs/hot/ www.nobrarequired.com www.day-without-rain.org/sandee/ www.sarswatch.org www.thecancerblog.com www.rdoc.org.uk www.cancergiggles.blog-city.com www.dipex.org
· For Guardian Unlimited's special report on weblogs, go to theguardian.com/online/weblogs