Travelling on the London Underground recently, I spotted a medical advert. “Healthy resolutions? Do everything you can to keep track of your body,” the poster declared, next to a picture of a smartphone displaying the results from “1,200 easy-to-use blood tests and health checks”.

In some senses, this is unremarkable: February, just as consumers’ New Year resolutions begin to wane, is probably a good time to sell health devices, and gadgets such as Fitbits and Jawbones have become wildly fashionable.

On another level, however, this poster also raises an issue that most of us have yet to consider. A couple of weeks ago, I conducted an interview at the World Economic Forum in Davos with Yuval Noah Harari, the Israeli historian and author of the best-selling books Sapiens and Homo Deus, which provide sweeping overviews of human history and its possible future. The exchange was bracing: Harari possesses a ferocious intellect, and his ideas have been highly influential, not just with the wider public but among world leaders. So much so that as we were waiting in the Davos green room before the interview, Angela Merkel, the German chancellor, made a point of greeting Harari; apparently she has read his books too.

Harari’s works might be on the radar of Merkel (and others) but his message is not remotely comforting. On the contrary, as he told the Davos elite, he believes that the appearance of the kind of medical trackers I saw advertised is one tiny sign of a staggering cyber revolution that could blow apart our western ideas about how democracy is supposed to work.

In recent years, we have become familiar with the idea that data on our finances, shopping and surfing habits are floating around in cyberspace. Belatedly, we have also started to pay more attention to who controls this data. At this year’s Davos, there were heated debates about the dominant role of companies such as Facebook, Google and Alibaba, not to mention the Chinese and US governments. Merkel also raised the subject in her speech.

For Harari, though, the really big issue is what will happen when computers start tracking not just our emails, messages and money but our bodies as well. “When you merge the revolution in infotech with the revolution in biotech, you get the ability to hack humans,” Harari told the Davos audience. He went on to argue that “the key invention that enables infotech and biotech to merge is the biometric sensor”(which can read fingerprints, for instance) and that “given enough such information and enough computing power, external systems can hack all your feelings, -decisions and opinions”.

This sounds utterly far-fetched now – those fitness gadgets are nowhere near that smart. But the technology is improving so rapidly that Harari thinks computers will eventually “know me better than I know myself”, since “humans often don’t know themselves very well”.

In some ways, this could be fantastic: biometric data could create personalised medical care and help us all become healthier and wiser about our bodies. But it could also be creepy, shattering any concept of “privacy”. Harari says he was 21 when he finally realised he was gay, “after several years of living in denial”, but imagines a future where even if a teenager was in denial about their sexuality, algorithms based on biotech would mean they couldn’t hide it from “Amazon, Alibaba or the secret police”.

This is the larger problem — the huge power given to whoever controls such data. Right now, most consumers do not reflect too deeply on this, tending vaguely to assume that they will be in control of their own information – this, after all, is the message implicit in the adverts for fitness gadgets. And this assumption may turn out to be true.

Don’t bet on it though, in a world where governments and tech companies are already hugely powerful. Harari expects that if current trends are left unchecked, the world may quietly slide towards a future of “data dictatorships”, as state or private entities use their access to our data to wield power.

Harari does not expect these data dictator- ships to emerge for “a couple of decades”. There is still time, then, for policy makers to start devising solutions to rein in the trajectory that he describes. But problems are already occurring: last week, it emerged that data from fitness tracking devices – published by Strava, a “social network for athletes”, as part of a global “heatmap” of its users’ activities – could reveal the locations of American military bases. And after listening to the debates in Davos, it seems that neither the politicians – nor, for that matter, Harari – yet have a clear idea about how to deal with these issues.

One thing is clear: it is unlikely that anybody will put this genie back in the bottle and throw the devices away – especially given the stunning potential health benefits that digital medicine can offer. As so often, innovation is a double-edged sword. Our biggest modern blessings are also potentially dangerous curses – and never more so than when wrapped up in the cultural label of “doing good”.