Anna (not her real name) lives an enviable life in many ways. She works out at least four times a week, and with her long blonde hair and thoughtfully applied make-up, she could easily pass for 10 years younger than her actual age of 58. Since taking early retirement from her management job she has enjoyed frequent jaunts abroad and though she recently ended a relationship with a younger partner, she hopes to have a new lover one day.

But if that makes her sound like she is living a sort of mid-life dream, think again. Anna is HIV positive.

Anna's story – that of an older, heterosexual British woman who has contracted HIV – is unusual, but she is certainly not alone. Over the past decade, the number of British-born women living with HIV has risen by 42%, with women over 50 making up the fastest-growing group. In 2011, for example, the latest year for which figures are available, the increase among 50-plus women was 19.2% (a jump from 2,914 to 3,474 cases, out of an overall figure of 73,659 including men and women).

Part of that increase is down to ageing: but Bernard Curtiss, support worker for older people with HIV at Thames Valley Positive Support, says much of it is due to the modern fiftysomething woman being more sexually active than ever before, but lacking the basic sexual nous of her younger counterparts. "Older women know they can't get pregnant any more – but they missed out on the messages about safe sex being about more than avoiding having a baby," he says.

So concerned is Thames Valley Positive Support, which is one of two HIV services in the UK funded by Comic Relief, that it published a report called Frisky – over 50? about the sexual health of this demographic. "What we found was that 30 years ago, women who got divorced or widowed in their 50s thought, that's it for me and sex," says Curtiss. "But these days, older women are using the internet, they're dating, and they're expecting to have a sex life. Which is all well and good, but they've missed out on the messages about safe sex, which have never been directed at them anyway."

That's certainly how it was for Anna. "I went to the Caribbean for a 'pick-me-up' holiday after splitting up from my husband," she says. "I met a lovely man and we started a relationship." At first they used condoms but, she says, not for long. "I knew I was past the age of getting pregnant, and we were pretty serious about one another. I returned to Berkshire where I live, but I went back to see him whenever I could."

When news came that he was ill, Anna hurried across the Atlantic to his bedside. "His family were at the hospital but none of them would tell me what was wrong," she says. "One day the doctor came in and was asked what he was being treated for, and I heard him say: 'Aids'."

Anna was horrified – but her thoughts were more about her lover than herself. It was only a few months later, after he had died, that she started to feel unwell. "I was always getting chest infections, skin rashes, and I had no appetite at all," she says. "It seems obvious now what was wrong, and I must have known somewhere inside, but I blocked it out. It was only when I collapsed and was taken to hospital that I realised I would have to tell someone." The first doctor she spoke to was astonished. "He looked at me, and I knew he was thinking: there's no way she's going to be an Aids sufferer."

But she was. Unlike her partner, though, Anna was lucky: living in the UK meant she was able to access the drugs needed to keep the virus under control. "It took a long time to recover, but eventually I did," she says. "I was in hospital for many weeks but they assured me that, even though I had got so ill, it wasn't a death sentence."

Five years on, she has clearly recovered physically; but emotionally it is still very tough. "There's no one I can share this with; I've had to make this journey almost entirely alone," she explains. Of her three grown-up children she has told only one, a son who lives nearby – "there had to be one person who knew, in case I got ill again". When she started a new relationship a couple of years ago, she told her partner before they had sex. "It was difficult, but I knew I couldn't have happen to him what had happened to me," she says. "But having said that, the drugs I'm taking do reduce the risk of me passing the virus on – and, needless to say, we always used condoms."

But otherwise, she has kept her HIV status entirely secret. Most people she knows couldn't begin to understand how this happened to her. "They'd see me as promiscuous and bad-living – they'd pass judgement. I'm a grandmother now, and I'm so not the sort of person you imagine when you think of someone living with Aids."

Thames Valley Positive Support's drop-in centre has provided the one "safe place" for Anna – and brought her new friends. "I've made friends who are gay, and I've made friends who are of African origin," she says. "So it's widened my social circle, and I've got very close to people I would never otherwise have met. But when I'm at home, in the rural village where I live, life can feel very lonely."

Thames Valley Positive Support is funded by Red Nose Day which takes place this Friday. For more information rednoseday.com