I met Tunde Balogh at the Catholic pilgrimage site of Lourdes, France. The 37-year-old, originally from Hungary, had been diagnosed with breast cancer a year earlier but refused conventional treatment.

“They’d cut off my breast, I didn’t want to do that,” she told me. Instead, she felt the answer was inside her. She tried reiki, reflexology, and eventually German New Medicine, which teaches that cancer is caused by emotional conflict. But none of it worked; the cancer soon spread to her bones. By the time she made it to Lourdes, her only hope was a miracle cure.

Let’s be clear: Claims that the mind can heal aren’t harmless. When made in the absence of evidence they raise false hope, and if people reject conventional treatment they need, they can die. That includes cancer patients, but less dramatic cases risk lives, too. Homeopaths regularly caution parents not to vaccinate their children against potentially fatal childhood infections, for example, and advise travelers against conventional drugs to protect against malaria.

Perhaps it’s not surprising, then, that skeptics react to any suggestion of healing thoughts as an evil threat to be stamped out, branding everything from placebo research to integrative medicine as “quackery.” But when researching my book, Cure: A Journey Into the Science of Mind Over Body, I came to the conclusion that this position isn’t supported by the science either. Although the mind isn’t a miracle cure—we will always need physical drugs and treatments—there is now overwhelming evidence that it drives biological changes that are crucial for physical health, influencing everything from pain to the immune system.

Our mental state has particularly dramatic effects when it comes to the symptoms we experience: things like pain, nausea, fatigue and depression. Playing a virtual-reality game eases pain in burn patients by as much as 50 percent (PDF) more than drugs alone, while research on placebos—fake treatments—tells us that psychological factors such as expectation and social interaction ease symptoms via biological changes very similar to those caused by drugs. Placebo painkillers trigger the release of natural pain-relieving chemicals called endorphins. Parkinson’s patients respond to placebos with a flood of needed dopamine. Breathing fake oxygen can reduce the levels of neurotransmitters called prostaglandins, which cause many of the symptoms of altitude sickness.

It might sound crazy that thoughts and expectations should have similar effects to drugs, but underlying many placebo responses is the simple principle that the symptoms we feel aren’t a direct, inevitable consequence of physical damage to the body. Such damage is important, of course, but ultimately our experience of it is created and controlled by the brain. If we feel stressed and alone, warning signals such as pain, fatigue, and nausea are amplified. If we feel safe and cared for (whether that means being surrounded by friends or receiving what we believe to be an effective medical treatment), our symptoms are eased.

This means that for many medical conditions, pouring ever more resources into physical drugs and interventions, while squeezing appointment times and cutting medical staff, may be counterproductive. One trial found that patients with irritable bowel syndrome (IBS) had much greater relief from their symptoms if the practitioner was warm and empathic rather then cold but polite—regardless of the treatment they received. Similarly, patients with acid reflux disease did dramatically better after an extended (42-minute) consultation with a physician, compared with a standard (18-minute) visit. In situations from back pain (PDF) to childbirth, patient outcomes depend not just on what drugs are prescribed but on how that care is delivered.

But this isn’t all. Because the brain controls physiological functions from digestion to the immune system, the mind doesn’t simply determine our subjective experience; it can be relevant for the physical progression of disease, too. These processes aren’t generally under voluntary control; we can’t “wish” ourselves better. But we can influence them, particularly by modulating our response to stress.

When you’re anxious your heart beats faster, for example, putting a greater strain on your cardiovascular system. This isn’t usually a problem, but in some circumstances it can be dangerous or even fatal. Natural disasters such as earthquakes sometimes kill as many people from heart attacks as from falling rubble. Trials show that during invasive medical procedures such as breast biopsies or destruction of tumors, people who feel negative or anxious beforehand suffer more complications (things like prolonged lack of oxygen, low or high blood pressure, post-operative bleeding or an abnormally slow heart rate). Relaxation techniques such as visualizing a safe place vastly reduce pain and anxiety during these procedures—as well as the rate of adverse events.

Feeling stressed can also have physical consequences on the gut. If we’re unhappy with toilet arrangements we might not go for days, whereas facing a challenge such as an exam or competition can cause us to empty our bowels. These processes exacerbate conditions such as IBS, and trials show that gut-focused hypnotherapy, which teaches patients to tackle stress and calm their digestive system—is a highly effective treatment. A course of such hypnotherapy reduces the sensitivity of the gut to pain, and while hypnotized, patients can alter their rate of gut contractions, something that we can’t normally do at will.

Third, stress triggers a branch of the immune system called inflammation: the body’s first line of defense against infection or injury. This is useful in an emergency but if triggered long-term by chronic stress, it disrupts healthy immune responses and eats away at the body’s tissues, making us more susceptible to infections, allergies, and auto-immune disease. And that doesn’t just mean eczema flareups or a few extra colds. Through its effects on the immune system, stress has also been shown to accelerate the progression of life-threatening conditions such as multiple sclerosis and HIV. Research into whether interventions that reduce stress can reverse these changes is only just beginning, but there’s some preliminary evidence that stress-management therapy can stall progression in MS, and that mindfulness meditation may slow HIV.

There’s even evidence that the mind plays a role in cancer. Inflammation clears damaged cells and promotes the growth of new blood vessels, which is useful for wound healing but also gives tumors the space and food supply they need to grow. In animal studies, stress hormones make a range of cancers spread faster, while patient trials suggest that stress-management interventions reduce inflammation, although the jury is still out on whether this feeds through into improved survival times.

Even if reducing stress doesn’t directly affect survival in cancer, however, there are other ways in which psychological approaches can improve physical prognosis. If easing the fatigue and nausea caused by chemotherapy helps someone stick to their treatment regime, it can improve survival. Meanwhile, social support helps patients to make better decisions. In one study, terminal cancer patients offered early palliative care chose to receive less aggressive treatment. They were less depressed, had a better quality of life—and they lived longer.

The mind can’t heal everything, and to reject physical treatments for life-threatening conditions is dangerous and misguided. But it’s clear that our mental state does have wide-ranging physiological effects that can affect health in many different ways, and which are relevant even to the most serious conditions such as cancer, multiple sclerosis, and HIV.

Skeptics are right to warn of exaggerated claims in mind-body medicine. But denying the role of the mind has its own risks. It pushes people—particularly those with direct experience of how it can help—away from science and toward the crackpot explanations of alternative therapists. And it blinds us to insights that could be hugely important for medicine. In Cure, I argue that we need to combine both approaches: to care for patients’ bodies and their minds.