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But what if was nothing of the sort? What if it was a physical illness that just happens to make people feel pretty lousy? Would that make it less of a big deal to admit to? Could it even put a final nail in the coffin of the idea that depression is all in the mind?

According to a growing number of scientists, this is exactly how we should be thinking about the condition. George Slavich, a clinical psychologist at the University of California in Los Angeles, has spent years studying depression, and has come to the conclusion that it has as much to do with the body as the mind. “I don’t even talk about it as a psychiatric condition any more,” he says. “It does involve psychology, but it also involves equal parts of biology and physical health.”

The basis of this new view is blindingly obvious once it is pointed out: everyone feels miserable when they are ill. That feeling of being too tired, bored and fed up to move off the sofa and get on with life is known among psychologists as sickness behaviour. It happens for a good reason, helping us avoid doing more damage or spreading an infection any further.

It also looks a lot like depression. So if people with depression show classic sickness behaviour and sick people feel a lot like people with depression – might there be a common cause that accounts for both?

The answer to that seems to be yes, and the best candidate so far is inflammation – a part of the immune system that acts as a burglar alarm to close wounds and call other parts of the immune system into action. A family of proteins called cytokines sets off inflammation in the body, and switches the brain into sickness mode.

Both cytokines and inflammation have been shown to rocket during depressive episodes, and – in people with bipolar – to drop off in periods of remission. Healthy people can also be temporarily put into a depressed, anxious state when given a vaccine that causes a spike in inflammation. Brain imaging studies of people injected with a typhoid vaccine found that this might be down to changes in the parts of the brain that process reward and punishment.

There are other clues, too: people with inflammatory diseases such as rheumatoid arthritis tend to suffer more than average with depression; cancer patients given a drug called interferon alpha, which boosts their inflammatory response to help fight the cancer, often become depressed as a side-effect.

As evidence like this continues to stack up, it’s not surprising that some people have shifted their attention to what might be causing the inflammation in the first place. Turhan Canli of Stony Brook University in New York thinks infections are the most likely culprit, and even goes as far as to say that we should rebrand depression as an infectious – but not contagious – disease.

Others aren’t willing to go that far, not least because infection is not the only way to set off inflammation. A diet rich in trans fats and sugar has been shown to promote inflammation, while a healthy one full of fruit, veg and oily fish helps keep it at bay. Obesity is another risk factor, probably because body fat, particularly around the belly, stores large quantities of cytokines.

Add this to the fact that stress, particularly the kind that follows social rejection or loneliness, also causes inflammation, and it starts to look as if depression is a kind of allergy to modern life – which might explain its spiralling prevalence all over the world as we increasingly eat, sloth and isolate ourselves into a state of chronic inflammation.

If that’s the case, prevention is probably the place to start. It’s not a great idea to turn off inflammation entirely, because we need it to fend off infections, says Slavich, but “lowering levels of systemic inflammation to manageable levels is a good goal to have”.