Fourteen years ago, I was paralyzed with Guillain-Barre syndrome, a rare autoimmune disease that left me unable to walk for nearly a year. Mine was an unusual case in its severity but the pathology of the condition is well understood: As in all autoimmune diseases, my immune system was behaving like an army gone rogue. Instead of judiciously protecting my body, my white blood cells were mistakenly destroying the myelin sheaths that coat my nerves, causing the nerve-muscle interconnections that I needed to stand and walk, or simply wriggle my feet, to go dark.

Courtesy of Ballantine Books Excerpted from The Angel and the Assassin: The Tiny Brain Cell That Changed the Course of Medicine by Donna Jackson Nakazawa. Buy on Amazon.

I began to think of these overactive immune cells as being like Pac-Men: 1980s video game characters crazily eating away at the crucial nerve connections that made my physical body mine—strong, capable, dependable me. My neurologist hoped that regular intravenous therapies would help to reboot my immune system so that my overvigilant white blood cells would cease their attack and my nerves would begin to regenerate on their own—perhaps not all of them, but enough to allow me to live a good life. In time, he turned out to be right. The human body can be miraculous that way.

Still, I had questions that my medical team could not answer. After losing the use of my legs, I’d also experienced some distinct and disquieting cognitive changes. For one thing, although I’d always been pretty even-keeled, I found myself facing a black-dog depression. The feeling was at times so oppressive that when I read Harry Potter aloud to my young children, I felt as if I’d been attacked by the “dementors,” those dark, sky-drifting ghouls who introduce a cloud of despair that steals a person’s happy thoughts and replaces them with bad ones.

Then, there were the memory problems. My six-year-old daughter would ask for help with first-grade math and I’d find my brain stuttering just to add seven and eight. Or I’d reach down to tie her shoes and find myself staring dumbly at the laces, struggling to remember how, exactly, it was done. My mood, memory, clarity of mind, word recall were different—my brain did not feel like my own. I could not shake the feeling that, just as my body had been altered, something physical had also shifted in my brain.

I was not, of course, the only patient with inflammation in my body who also complained of a change in mental well-being. Such patient stories had even led some epidemiologists to begin to associate inflammatory diseases to brain-related symptoms. In 2008, researchers reported that patients with multiple sclerosis were several times more likely to suffer from depression and bipolar disorder. A 2010 analysis of 17 studies showed that 56 percent of patients with lupus—which manifests in inflammation in the organs of the body—reported cognitive or psychiatric symptoms. Patients recently hospitalized with bacterial infections were 62 percent more likely to develop depression, bipolar disorder, and memory issues.

Several case studies in the scientific literature showed a curious link between disorders in the bone marrow—where most of our body’s immune cells are “born”—and schizophrenia. In one case study, a patient who received a bone marrow transplant from his brother, who suffered from schizophrenia, also developed schizophrenia a few weeks after the transplant. In another, a young man with schizophrenia and acute myeloid leukemia received a bone marrow transplant from a healthy donor, and both his cancer and his schizophrenia resolved.

Yet, as compelling as this research was, it just did not make scientific sense at that time that being sick in the body could be connected to, much less cause, illness in the brain. The philosopher Descartes first put forth this concept known as mind-body dualism—the idea that the workings of the brain are divorced from the mechanisms of the body—during the Enlightenment. This ruling out of a biological link between immune health and brain function was due, however, more to the work of 19th-century scientists than early philosophers. At the base of the brain sits a dense constellation of cells known as the blood-brain barrier. These cells are so tightly packed around the blood vessels that lead to the brain that they block immune cells from the bloodstream from ever rising into the brain. The inviolate nature of the blood-brain barrier has long been seen as ample proof that the brain is off-limits to the body’s immune system—or “immune-privileged.” Since the first modern day med school textbooks were penned, they’ve taught a simple anatomical fact: The immune system rules every bodily organ except the brain, and physicians have operated under the assumption that body and brain function as church and state entities.