Imagine the response to a 139% increase in syphilis among teenagers in just eight years: the articles in the rightwing press condemning promiscuous adolescents, the calls by liberal columnists for better sex education in schools, the slightly watching-your-Dad-dance embarrassing "yoof" campaigns launched by concerned New Labour focus groups. At least it would be a vocal and varied response to a serious public health risk.

But you won't be seeing any of that this week, because the 139% increase isn't among teenagers, but 45-64 year-olds. And, as anyone who has worked for a charity can tell you, some causes just aren't sexy. Internet dating, Viagra and divorce among the over 50s have all been pointed to as reasons for this increase, and Julie Bentley, chief executive of FPA (formerly the Family Planning Association), has told the Independent "it's imperative that we move away from the equation that sexual health equals young people."

No one is suggesting that under 25s aren't still the main group to be targeted with sexually transmitted infection (STI)-prevention schemes: in 2007, the Guardian reported that teenagers made up 40% of females infected with gonorrhoea. But the new statistics on older patients published by the Health Protection Agency are alarming and, for many, unexpected. The overall rate of infections among over 45s more than doubled within eight years, from 16.7 per 100,000 population to 36.3 per 100,0000. And within the over 45 group, it is actually men and people aged 55 to 59 who are most likely to have an STI.

I have a feeling the findings that will make many uncomfortable are the statistics relating to sex and the over 55s. We can just about handle sexual activity among people in their forties and early fifties, at least when they look like Kim Cattrall, but, to put it bluntly, no one wants to think about their grandparents having sex. Yet, there have been reports of STI infections among people in their seventies – those who were teenagers before sex was, supposedly, invented in 1963.

I have to admit I was a little surprised when my friend who works in a sexual health clinic in Canada first told me about a mass outbreak of gonorrhoea in a care home. Of course, it's a little bit funny, and that's OK – lots of things to do with sex are a little bit funny. But it's also serious, because we're talking about an epidemic. And since our nervous laughter reveals how we've mentally de-sexualised vast swathes of the population – not just the elderly, but also the disabled, the obese, all those who aren't lithe and nubile – it is clear that even the most liberal of us have trouble with the idea that we all feel the full range of human emotions, including sexual ones.

Until we begin to tackle this mindset, STIs will continue to spread among the elderly.