A new report suggests that HIV infection and prevalence rates are surprisingly high among older people, who seem to have been completely ignored by research and surveillance, perhaps because we don't assume they are at risk. However this also means that many cases of HIV among the over 50s go undiagnosed. The authors said more research is needed to identify the risk factors for HIV infection among older people.

These are the conclusions of scientists working for and on behalf of the World Health Organization (WHO) whose findings were published in the Bulletin of the World Health Organization earlier this week.

There is little data on how many people over the age of 50 are infected with HIV (ie prevalence), as this age group is rarely included in health surveys.

Data on HIV incidence rates among older people is also scarce (incidence is new cases per year whereas prevalence is how many people have a disease).

While developing countries have limited case reporting systems, the authors wrote that as far as identifying HIV rates in older people, industrialized countries aren't much better. However, what data there is suggests incidence of HIV in older people is rising, mostly from information gathered in the US.

To start shining some light in this rather dark corner of our public health awareness, the authors calculated incidence and prevalence of HIV among older people using what information they could get hold of.

For example, using the UNAIDS estimate of HIV cases and population numbers by country, they found a consistent pattern: HIV prevalence in people aged 50 and over was between 25 and 33 per cent of that among people aged 15 to 49.

The authors wrote that after some debate, most of them decided this was a surprising finding.

Figures from Africa show that in many regions, a second peak of HIV incidence appears in the older age group.

When the authors looked in the literature for explanations of how older people become infected with HIV they found only one small study.

We know very little about the sexual habits of older people in the developing world. There is some evidence that many older people throughout the world are sexually active, although interest and frequency of vaginal intercourse does wane with age, wrote the authors.

Much research shows that older people are less likely to practise safe sex compared to younger age groups, and there is some evidence from industrialized countries that use of erectile dysfunction drugs is linked to risky sexual behaviour, and there have been debates about whether men who are HIV-positive should be given these drugs.

If the scant data is to be believed, and the main cause of HIV infection in older people is sex, but they are not having intercourse as frequently, then this suggests older people are more vulnerable to the virus: their chance of becoming infected each time they have sex is much higher than for their younger couterparts.

One biological reason could be that the wall of the vagina gets thinner as women get older, making it easier for the virus to get into the bloodstream.

However, the authors emphasized that while sexual activity was probably the most likely way, more research was needed to find out the relative risks of the different ways older people acquire HIV.

There is however, a consistent failure among health professionals to consider HIV as a cause of illness among older patients, said the researchers.

Older people diagnosed with HIV progress to AIDS much faster, and this could be partly because of age but also because they are not diagnosed as early as their younger counterparts, who are also more likely to be screened for the virus.