In 1989, University of Cambridge psychiatrist Ed Bullmore, then a physician-in-training, examined a woman unable to walk from destroyed collagen and bone in her knees. That, plus painfully swollen joints in her hands, indicated rheumatoid arthritis—an inflammatory disease. After Bullmore asked the usual questions to reach the diagnosis, he inquired about something else: how she was feeling emotionally.

In the past two decades, scientists have begun to realize that the link isn't merely a byproduct of living with a difficult disease. Researchers—like Bullmore, who wrote a book called _The Inflamed Mind—_have been uncovering a more meaningful and sinister connection between inflammation and mental health. Inflammation seems to directly cause mental health issues, while at the same time, stress and mental health issues themselves provoke inflammation—creating a dangerous feedback loop.

These disorders of chronic inflammation are steadily increasing . This is troubling not only because they are a leading cause of death, but because, as in the case of Mrs. P, there's an association between these disorders and mental health issues, which are also on the rise .

Inflammation is a natural and necessary function of the immune system. It's what happens when your body activates an army of different molecules to protect itself from an outside threat—say, to fend off a virus or heal a wound. But when inflammation persists, it can wreak havoc. Autoimmune and inflammatory disorders like arthritis, asthma, inflammatory bowel disease, Type 1 diabetes, and allergies happen when the body's immune system doesn't know when to call back the troops.

When he told his supervisor, the senior doctor didn't find it very exciting. Having a chronic inflammatory disease would likely make anyone depressed, he said. "It did not occur to either of us that it might originate in the body," Bullmore wrote. "That Mrs. P might be depressed, not because she knew she was inflamed, but simply because she was inflamed."

"Over the course of the next 10 minutes or so, she quietly but clearly told me that she had very low levels of energy, nothing gave her pleasure anymore, her sleep was disturbed, and she was preoccupied by pessimistic and guilty thoughts," he wrote in a 2018 Medium essay.

One single risk factor will never explain the entirety of psychiatric problems, wrote Chuck Raison, a psychiatrist at the University of Wisconsin-Madison, in a special report on the topic in Psychiatric Times. But "inflammation turned out to be a common denominator and likely risk factor for every manner of psychiatric disturbance, from schizophrenia to obsessive compulsive disorder, from mania to depression," he wrote.

Christopher Lowry, a behavioral neuro-endocrinologist at University of Colorado Boulder, envisions a future where an immunization could be given to protect against inflammation-caused mental health disturbances. In a series of studies over the past decade, Lowry and his collaborators have shown in animal models that a particular soil bacteria, Mycobacterium vaccae (or M. vaccae), can reduce inflammation and the troubling behavioral symptoms that come with it.

Inflammation seems to cause mental health issues, and mental health issues themselves provoke inflammation, creating a dangerous feedback loop.

One reason runaway inflammation happens is that the immune system has lost its way. A surprising potential explanation for why this can happen is emerging: Our immune systems could be missing exposure to some crucial things that aid in its healthy development: bacteria and viruses, and parasitic worms—the very things it is designed to ward off. The loss of exposure to these microbial organisms due to our modern diets, medications, and lifestyles could be producing naive and untrained immune systems that have a tendency to go haywire.

The immune system can't fire off like a loose cannon at every foreign cell, it has to decide when something is a threat and when it's not. That's why humans and other mammals have both an innate and an adaptive immune system, said Graham Rook, a professor of medical microbiology and immunology at University College London. The latter learns what to target and when, but to do so, it needs data.

"It's is like a computer," Rook said. "The question is, where does the data come from? It comes in part from the microbiota, or the organisms in the gut. It also comes from outside infection and other microbes we are exposed to, especially in early life."