Local authorities will inherit plenty of challenges when they take responsibility for public health in April, but they have a stellar track record of innovation in this arena.

In the 19th century, for instance, local councils' Victorian counterparts managed to halve infant mortality through improving water and housing, and by eradicating the foul, infectious sludge-loos known as "privy middens".

Disposing of human waste instead of letting it flow in open sewers seems obvious today, but the new public health directors will need to employ similarly radical thinking if they are to tackle today's plagues of obesity and sexually transmitted infections. To do so, they will need to turn to the internet.

The internet is hyper-personalised and made up of countless interlocking communities: people bound by common religion, geography, political affiliation, parental status or favourite member of One Direction. Online retailers take advantage of this by slicing the web through behavioural and (increasingly) semantic targeting, allowing them to target groups of individuals with messages that appeal to them.

Similarly, an entry-level strategy for public health would involve engaging online communities made up of people with the same medical condition.

But the potential to use online communities for public health goes far beyond this. What if we didn't wait for people to self-define as having a particular condition but targeted them before they knew they had it? What if we could use the internet to conduct interventions – not just education, but also testing and treatment – on people who are at high risk?

Some providers are already starting to do just this. Operated by the Chelsea and Westminster hospital's sexual health centre in Soho, 56 Dean Street, in conjunction with the remote healthcare specialist DrThom and the gay dating site Gaydar, the Dean Street at Home initiative uses the principles of online marketing to diagnose HIV efficiently through careful targeting.

Gaydar brings together people with a relatively high risk of having or contracting HIV, so users are sent messages offering a free risk assessment. DrThom then offers a free HIV home-sampling kit to participants via the post so they could give a sample from the privacy of their own homes.

These are then sent off to the laboratory and the test results sent back to DrThom. Those who test negative are informed by SMS; those whose sample proves "reactive" are contacted by the NHS doctors of 56 Dean Street and offered further advice and testing.

The results have proved dramatic. Of the 4,500 people invited through Gaydar, 132 requested HIV test kits, many of whom had never been tested before. The overall rate for positive tests was 2%, far above the 0.2% considered to be cost-effective.

Offline programmes tend to be far less successful. Testing all new registrants at a GP surgery or acute admissions at hospitals, for instance, have their benefits – reducing stigma is one – but are much more expensive in terms of identifying one positive.

The online model of Dean Street at Home, however, targets the people most likely to need testing. They are only offered the test once that has been determined.

There is a compelling case for using this approach with other public health challenges, other conditions, other high-risk groups and other websites. If we can understand the risk profile of visitors to non-medical websites, then we can target them with public health messages in a more effective and efficient way.

The Victorian social reformers might be a bit taken aback by our approach, but I like to think they'd approve.

Rachel Carrell is chief executive of DrThom

• This article was corrected Thursday 7 February to change the caption to say: "A Soho health centre targeted gay men for HIV testing online". It previously said Aids testing, which was incorrect.

The Guardian Health Professionals Network will be running the fourth Public Health Dialogue Thursday 7 February at 6.30pm. You can follow debate at the event via the #GdnPublicHealth hashtag on Twitter.

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