Infectious Madness: The Surprising Science of How We "Catch" Mental Illness


Gazing into the night sky with its seemingly numberless stars evokes our sense of infinity, but if you seek the ultimate multitude, look closer to home. What lies at our feet and within us dwarfs the heavenly spectacle, yet we need our imaginations—or a powerful microscope—to see it: microscopic bugs, not stars, dominate the galaxies. The earth alone holds five million times more microbes than there are suns in the universe. It is home to five nonillioninfinitesimal beings—that’s a 5 followed by 30 zeros.

Five million bacteria teem in every teaspoonful of seawater, accompanied by fifty million viruses. This makes viruses the most common life form in the seas, and no wonder: viruses infect most other living organisms, including bacteria.

Microbes do more than infect us, however; they are us, in the sense that we harbor more microbes than human cells. Your intestines alone provide a home for one hundred trillion viruses, fungi, protozoans, and—mostly—bacteria. These single-celled guests outnumber your cells ten to one.

Microbes thickly coat our skin, eyes, and genitals and cover the surfaces of our mouths; they specialize in specific areas of the body. Staphylococci colonize the skin, and lactobacilli coat the vagina. And that’s just on the surface; ten thousand different species of organisms thickly populate your gut. Just as our genes make up our genomes, these microbial fellow travelers make up our microbiomes, which constantly adjust in type and numbers on different sites on the body and different sites on the globe.

And our health, including our mental health, changes with them.

Your microbiome has an astonishing power to keep you healthy—or ill. From the beginning, internal microbes guide your immune system’s development. Your gut also possesses its own “brain.” It houses a network, dubbed the enteric nervous system, or ENS, that contains a thousand times more neurons than your brain does. Its weight is twice that of your brain and it sends neurotransmitters that help direct your brain’s activities.

During birth we acquire microbes from our mothers that confer immunity and may dictate our future health, from struggles with weight to a propensity to schizophrenia. As we grow, we acquire more pathogens and beneficial microbial “friends” that tip our odds of developing or avoiding everything from ulcers to heart disease to cervical cancer to obsessive-compulsive disorder.

The relationship between disease and microbes was first proposed in the seventeenth century, but the evidence and basic standards for the proof of infectious-disease causation weren’t laid down until 1883, when the German bacteriologists Robert Koch and Friedrich Loeffler provided the first evidence that tiny, invisible microbes enter the body and cause diseases; this is germ theory.

The microscope enabled scientists to see the pathogens, document them, and, in doing so, disprove popular beliefs, such as that sinful behavior invites illness or that poisonous vapors called miasmas cause disease.

The uncontrollable dancing movements of St. Anthony’s fire, once ascribed to satanic influence, are now known to result from Claviceps purpurea, a fungus that infects rye. Malaria is now known to be caused not by bad air but by a single-celled plasmodium, a parasite of the female Anopheles mosquito.

Contemporary researchers on five continents continue to unmask microbial roots of illness, and they now recognize that events as seemingly trivial as a sore throat or a case of measles—or even the flu—can breed anorexia, Tourette’s, obsessive-compulsive disorder, or schizophrenia. Researchers in the field estimate that infectious organisms cause from 10 percent to 75 percent of some serious mental disorders.

In 1997, I glimpsed the extent to which mental illnesses are connected to infection when I happened upon an Italian medical-journal article that linked schizophrenia to bornavirus, which causes a fatal encephalitis in Central European sheep and horses. It asked whether humans acquire the virus from horses and whether such infections can cause schizophrenia. The article found a strong correlation between infection and illness, but no proof. Thus, it was inconclusive, and so were subsequent studies, as I learned when I called the investigators.

I was disappointed, but my curiosity was piqued as I searched for evidence of causal connections between infection and mental illness. I quickly found them, but many were lodged in the past. General paresis, cases of which once filled one of every five New York City asylum beds, is caused by a familiar disease: syphilis. When scientists discovered that penicillin cured syphilis, they also discovered a cure for this common mental disease. Now one must travel to the developing world to see a case.

In 1997, I learned of Paul Ewald, a visionary evolutionary biologist whose work describes a second wave of germ theory. He has elegantly argued for the unperceived importance of infection as an explanation of much human disease. Bacteria, viruses, fungi, and other infectious agents are responsible for many of the illnesses that we have long ascribed to genetics, behaviors, and even personality types. Cervical cancer, for example, was long chalked up to sexual immoderation but is now known to be triggered by certain strains of the human papillomavirus, just as the hepatitis C virus causes hepatitis C. Ninety percent of ulcers, which were long attributed to unmanaged stress and treated with milk, antacids, and the acid-lowering drug Tagamet, are actually caused by a bacterium, Helicobacter pylori, although stress may impair an ulcer’s healing. Many heart attacks, long ascribed to aggressive, hostile, type A personalities, are now recognized as the legacy of the bacterium Chlamydophila pneumoniae as well as various gut bacteria.

In 1997 I also discovered the work of Dr. Susan Swedo, who proposed an intriguing syndrome fingering Group A streptococcal bacteria, or GAS bacteria, as the culprit in children who developed symptoms of anorexia, obsessive-compulsive disorder, or Tourette’s in the wake of strep throats. She was actively seeking proof and a mechanism in human studies with scores of adolescents, many of whom had been brought to her clinics at the National Institute of Mental Health by their worried parents. I reported on these exciting developments in Psychology Today, but aside from Swedo’s fledgling human studies, I found little contemporary evidence, just tantalizing correlations between infections and madness.

I periodically looked into the state of research linking microbes and mental disorders, and in 2013, I realized that it was burgeoning. With the acknowledgment of epigenetics, scientists moved away from exclusively genetic models of disease, including mental disease, and this made it easier to contemplate microbial causes and risk factors.

The pioneering research of scientists like Michael Gershon and Martin J. Blaser laid the groundwork for an emphasis on gut microbes that resulted in 2008’s Human Microbiome Project, a $115 million enterprise that sought to discern the microbial causes of health and illnesses, including depression, autism, and obesity.

Since the early 1970s, when Freud’s theories of mental illness were ascendant, prescient scientists like E. Fuller Torrey, director of the Stanley Medical Research Institute, and Robert Yolken, of Johns Hopkins, had rejected the belief that schizophrenia and other psychoses were exclusively the result of social and psychological dynamics. Instead, they had looked for answers in biology, specifically in microbial assaults on the immune and nervous systems. By 2013, mental-illness researchers had largely abandoned Freud to join the duo in exploring neurophysiology.

As a result of this sea change in research directions, we are approaching critical mass: a paradigm shift that replaces psychosocial factors with biological ones as the cause of mental illness. Most (not all) involved researchers think that microbes constitute just one risk factor; genetics, stress, psychological factors, and social dynamics are still important. In fact, most experts who hazard an informed guess about their relative importance suggest that infections cause 10 to 15 percent of mental disease. That may sound like a small number at first, but it is quite significant, especially when we consider, within that statistic, the many lives lost through suicide or early death and the even greater number of lives lost to profound disability.

Moreover, Yolken reminds us that immense numbers of mentally ill in poor and developing nations go undiagnosed, and we are not even aware of a greater number of microbes that undoubtedly exist in such areas; it is a mathematical certainty that some of them pose mental-health threats.

This book traces the growing evidence of microbial triggers of mental disease in infants, adolescents, adults, and people in the developing world. In describing the infinite variety of pathogenic mental disorders, it also interrogates the nature of proof, as opposed to mere correlation, and proposes that traditional mechanisms for establishing proof must be supplemented by modern tools and strategies. It examines the equally outmoded and simplistic notion of the “war” between man and his microbial hangers-on and suggests that our seek-and-destroy approach to pathogen control must be replaced by more nuanced strategies; we are involved in a chess game, not a brute battle to the death.

This book urges readers to employ the reasoning scientists have applied to physical illness to microbial mental illness. I’ll show through historical examples that we have been loath to follow the facts that establish microbial causes and that our biases and antiquated habits of thought have resulted in our clinging to scientifically untenable and ineffective theories and treatments that have cost many their sanity and their lives.

Not only do microbes play a surprising role in our tastes and preferences—some acquired tastes seem related to our microbial exposure, as I’ll explore—they also shape our societies. Infectious Madnessdiscusses research demonstrating how microbes influence our collective behavior, shedding light on issues that go far beyond individual mental health. It looks at how the poor and medically underserved suffer far worse mental health than the rest of the population, in part because neither their pathogens nor their mental ailments receive appropriate scientific treatment. As it turns out, microbes shed light on some of the most mysterious and vital questions we face: Why are some societies more xenophobic than others? Why do some peoples tolerate or even encourage stranger violence, such as lynching, the Holocaust, or ethnic genocide in Bosnia and Rwanda?

In short, Infectious Madness endeavors to relate through the prescient work of visionary scientists how microbes rule not only the world, but also our minds.



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