We swim in oceans of quackery. The media is flooded with misinformation about health and pseudo-diagnoses based on vibes rather than evidence. Books awash with error and supposition swamp our charts, penned by people uniquely unqualified to write them. Our ears are filled with popular podcasts claiming health benefits but really just peddling unregulated dietary supplements. And Robert Kennedy Jr, a man who has spent a lifetime spewing antivaccine jibber-jabber, is now US secretary of health. Vaccination is arguably the most successful health intervention in history (with the possible exception of sanitation), and now more than ever we should be basking in the fact that a global pandemic was brought to a close by safe and effective vaccines.

But here’s the conundrum: medical diagnoses are on the rise across the board, in many cases dramatically, and this is fuel for the medical disinformation industry. The most obvious example is autism, the incidence of which has shot up in a couple of decades, correlated with, but not caused by, an increase in vaccination. Cancer diagnoses are also up. A lot more people seem to have ADHD these days, which was barely around when I was at school. And millions now endure long Covid, a disease with a bucket of symptoms that did not exist at all five years ago.

Some pessimists will have you believe that this all reflects an increasingly moribund society, one suffering from pathological decline – modern life is making us sick, goes the ill-informed mantra. Thankfully, Suzanne O’Sullivan is here to help, and her voice deserves amplification. Thirty years a doctor, 25 a neurologist, her excellent books occupy a space once dominated by Oliver Sacks, where individual tales of disease and distress reveal broader truths about science, medicine and people.

One way to rebut the sick society thesis is to argue that diagnoses have increased because we are now more scientifically literate; obliviousness has been replaced by greater awareness and molecular precision, and we are in fact healthier than ever because of this newfound exactitude. But in her outstanding new book O’Sullivan offers a third possibility; that variance in bodily and mental health is being unnecessarily medicalised and pathologised: “We are not getting sicker – we are attributing more to sickness.”

She describes a trinity of “overs”. Overdiagnosis, where a medical problem is treated when treatment might not be needed; overmedicalisation, where non-medical behaviours are turned into the business of doctors; and underlying both, overdetection: we are ever better at identifying signals of disease, sometimes earlier than necessary, when those indicators may not end up presaging the disease itself. For example, some studies have shown that early screening programmes for cancers may result in arduous treatment when cancer itself was not inevitable.

Alongside balanced analysis of the epidemiological data on prostate and breast cancer, O’Sullivan examines the growth in behavioural conditions such as autism and ADHD. The tone is not sneering or dismissive, as debunkings of bad science so often can be. O’Sullivan is instead full of compassion, care and grace.

She presents case studies of pseudonymised patients, and each one highlights the delicacies involved. Poppy is autistic. She is clear that she is not a person with autism, as she does not carry it round in a bag – it is part of the way she is. Her diagnosis helps her understand herself, and maybe helps others understand her behaviour. In the past, Poppy might have been a kid in your class who was a bit odd, and was bullied for her unusualness. Now, the label helps her process being atypical. In children diagnosed with moderate or mild autism, there is plenty of evidence that diagnosis can be beneficial, but there is emerging evidence that it also comes with significant harms: a lowering of self-esteem, a reluctance to attempt things because of a perception that being autistic will make them harder. In adults, there is scant evidence of either benefits or harms from the diagnosis itself.

O’Sullivan devotes a chapter to Lyme disease and long Covid, two ailments with a notoriously broad set of symptoms that can be life-altering. The cause of Lyme disease is well known, a bacterial infection transmitted by deer ticks, but patients endure the full spectrum of mistreatment, from disbelief to huge overdiagnosis on the basis of limited biological evidence. The misdiagnosis rate is estimated as a hefty 85%, facilitated by doctors who may not understand the tools they are using, supplied by a very small set of companies who some might accuse of exploiting people desperate for answers about their nebulous but undeniable problems. Long Covid, similarly, has effects that aren’t limited to any one organ or anatomical system. I can tell you, as someone who’s had it, that it is also very real. Uniquely, though, diagnosis of long Covid has been led by the public, often via social media. O’Sullivan argues that this has complicated scientific definition and methodical study.

These are incredibly difficult areas to explore. Medicine, of course, exists to help people. It is about identifying and treating disease, but also about preventing people getting sick in the first place. The factors influencing those seemingly simple aims are extremely complex, encompassing an ever-evolving scientific evidence base and ever-changing diagnostic criteria. The meaty manuals that describe mental health issues (the most well known of which is the Diagnostic and Statistical Manual of Mental Disorders or DSM) have chequered histories. They remain widely used but heavily criticised – for the unreliability of their diagnoses, for the arbitrary lines that divide what is considered normal from mental illness, and for cultural bias.

O’Sullivan is brave to take this subject on, and she hits the target. I have little tolerance for tedious old men droning on about how “in our day, you just got on with things”, or “now everyone’s got autism/ADHD”, or worst of all that young people today are delicate snowflakes, medicalising everything and blaming everyone else. Apart from anything, this sentiment has been expressed by every older generation for thousands of years. So how do you take on a real set of problems in medicine, concern about which can be seen as conservative-coded, without getting into bed with the vibes-based bores who will bang their hammy fists on tables in prejudiced agreement? The answer is: carefully. O’Sullivan is an excellent, fluid writer, and an eloquent speaker, but I’m bracing myself for braying allyship from rightwing broadcasters during her very well-deserved media appearances.

In a world where medical misinformation and disinformation flourish, and people die as a result, it takes courage to counter them without pandering to stereotypes. But that is what The Age of Diagnosis does so well. Ignore the subtitle – “Sickness, Health and Why Medicine Has Gone Too Far’ – publishers’ sensationalism that belies an otherwise compassionate and measured book. Its overall message is clear: diagnosis is a tool to be wielded with the utmost caution, and tolerance for difference and for imperfection can go a long way in keeping us healthy.

The Age of Diagnosis by Suzanne O’Sullivan is published by Hodder (£22). To support the Guardian and Observer, order your copy at . Delivery charges may apply.
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