It is difficult to read any historical account of smallpox without encountering the word filth. In the 19th century, smallpox was widely considered a disease of filth, which meant that it was largely understood to be a disease of the poor. According to filth theory, any number of contagious diseases were caused by bad air that had been made foul by excrement or rot. The sanitary conditions of the urban poor threatened the middle class, who shuttered their windows against the air blowing off the slums. Filth, it was thought, was responsible not just for disease, but also for immorality. “Unclean! Unclean!” the heroine of Dracula laments when she discovers she has been bitten by the vampire, and her despair is for the fate of her soul as much as the fate of her body.

Filth theory was eventually replaced by germ theory, a superior understanding of the nature of contagion, but filth theory was not entirely wrong or useless. Raw sewage running in the streets can certainly spread diseases, although smallpox is not one of them, and the sanitation reforms inspired by filth theory dramatically reduced the incidence of cholera, typhus, and plague. Clean drinking water was among the most significant of those reforms. The reversal of the Chicago River, for instance, so that the sewage dumped in the river was not delivered directly to Lake Michigan, the city’s drinking-water supply, had some obvious benefits for the citizens of Chicago.

Long after the reversal of that river, the mothers I meet on the beaches of Lake Michigan do not worry much over filth. Most of us believe that dirt is good for our kids, but some of us are wary of the grass in the parks, which may or may not have been treated with toxic chemicals. The idea that “toxins,” rather than filth or germs, are the root cause of most maladies is a popular theory of disease among people like me. The toxins that concern us range from particle residue to high-fructose corn syrup, and particularly suspect substances include the bisphenol A lining our tin cans, the phthalates in our shampoos, and the chlorinated Tris in our couches and mattresses.

I already practiced some intuitive toxicology before my pregnancy, but I became thoroughly immersed in it after my son was born. As long as a child takes only breast milk, I discovered, one can enjoy the illusion of a closed system, a body that is not yet in dialogue with the impurities of farm and factory. Caught up in the romance of the untainted body, I remember feeling agony when my son drank water for the first time. “Unclean! Unclean!” my mind screamed.

“He was too pure,” a Baltimore mother said of her son, who developed leukemia as an infant. His mother blamed the pollutants in vaccines for his illness, and herself for allowing him to be vaccinated. Fears that formaldehyde from vaccines may cause cancer are similar to fears of mercury and aluminum, in that they coalesce around miniscule amounts of the substance in question, amounts considerably smaller than amounts from other common sources of exposure to the same substance. Formaldehyde is in automobile exhaust and cigarette smoke, as well as paper bags and paper towels, and it is released by gas stoves and open fireplaces. Many vaccines contain traces of the formaldehyde used to inactivate viruses, and this can be alarming to those of us who associate formaldehyde with dead frogs in glass jars. Large concentrations are indeed toxic, but formaldehyde is a product of our bodies, essential to our metabolism, and the amount of formaldehyde already circulating in our systems is considerably greater than the amount we receive through vaccination.

As for mercury, a child will almost certainly get more mercury exposure from her immediate environment than from vaccination. This is true, too, of the aluminum that is often used as an adjuvant in vaccines to intensify the immune response. Aluminum is in a lot of things, including fruits and cereals as well as, again, breast milk. Our breast milk, it turns out, is as polluted as our environment at large. Laboratory analysis of breast milk has detected paint thinners, dry-cleaning fluids, flame retardants, pesticides, and rocket fuel. “Most of these chemicals are found in microscopic amounts,” the journalist Florence Williams notes, “but if human milk were sold at the local Piggly Wiggly, some stock would exceed federal food-safety levels for DDT residues and PCBs.”

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The definition of toxin can be somewhat surprising if you have grown accustomed to hearing the word in the context of flame retardants and parabens. Though toxin is now often used to refer to man-made chemicals, the most precise meaning of the term is still reserved for biologically produced poisons. The pertussis toxin, for example, is responsible for damage to the lungs that can cause whooping cough to linger for months after the bacteria that produce it have been killed by antibiotics. The diphtheria toxin is a poison potent enough to cause massive organ failure, and tetanus produces a deadly neurotoxin. Vaccination now protects us against all these toxins.

Toxoid is the term for a toxin that has been rendered no longer toxic, but the existence of a class of vaccines called toxoids probably does not help quell widespread concerns that vaccination is a source of toxicity. The consumer advocate Barbara Loe Fisher routinely supports these fears, referring to vaccines as “biologicals of unknown toxicity” and calling for nontoxic preservatives and more studies on the “toxicity of all other vaccine additives” and their potential “cumulative toxic effects.” The toxicity she speaks of is elusive, shirting from the biological components of the vaccines to their preservatives, then to an issue of accumulation that implicates not just vaccines, but also toxicity from the environment at large.

In this context, fear of toxicity strikes me as an old anxiety with a new name. Where the word filth once suggested, with its moralist air, the evils of the flesh, the word toxic now condemns the chemical evils of our industrial world. This is not to say that concerns over environmental pollution are not justified—like filth theory, toxicity theory is anchored in legitimate dangers—but that the way we think about toxicity bears some resemblance to the way we once thought about filth. Both theories allow their subscribers to maintain a sense of control over their own health by pursuing personal purity. For the filth theorist, this means a retreat into the home, where heavy curtains and shutters might seal out the smell of the poor and their problems. Our version of this shuttering is now achieved through the purchase of purified water, air purifiers, and food produced with the promise of purity.

Purity, especially bodily purity, is the seemingly innocent concept behind a number of the most sinister social actions of the past century. A passion for bodily purity drove the eugenics movement that led to the sterilization of women who were blind, black, or poor. Concerns for bodily purity were behind miscegenation laws that persisted for more than a century after the abolition of slavery, and behind sodomy laws that were only recently declared unconstitutional. Quite a bit of human solidarity has been sacrificed in pursuit of preserving some kind of imagined purity.

If we do not yet know exactly what the presence of a vast range of chemicals in umbilical cord blood and breast milk might mean for the future of our children’s health, we do at least know that we are no cleaner, even at birth, than our environment at large. We are all already polluted. We have more microorganisms in our guts than we have cells in our bodies—we are crawling with bacteria and we are full of chemicals. We are, in other words, continuous with everything here on Earth. Including, and especially, each other.

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One of the appeals of alternative medicine is that it offers not just an alternative philosophy or an alternative treatment but also an alternative language. If we feel polluted, we are offered a “cleanse.” If we feel inadequate, lacking, we are offered a “supplement.”  If we fear toxins, we are offered “detoxification.” If we fear that we are rusting with age, physically oxidizing, we are reassured with “antioxidants.” These are metaphors that address our base anxieties. And what the language of alternative medicine understands is that that when we feel bad we want something unambiguously good.

Most of the pharmaceuticals available to us are at least as bad as they are good. My father has a habit of saying, “There are very few perfect therapies in medicine.” True as it may be, the idea that our medicine is as flawed as we are is not comforting. And when comfort is what we want, one of the most powerful tonics alternative medicine offers is the word natural. This word implies a medicine untroubled by human limitations, contrived wholly by nature or God or perhaps intelligent design. What natural has come to mean to us in the context of medicine is pure and safe and benign. But the use of natural as a synonym for good is almost certainly a product of our profound alienation from the natural world.

“Obviously,” the naturalist Wendell Berry writes, “the more artificial a human environment becomes, the more the word ‘natural’ becomes a term of value.” If, he argues, “we see the human and the natural economies as necessarily opposite or opposed, we subscribe to the very opposition that threatens to destroy them both. The wild and the domestic now often seem isolated values, estranged from one another. And yet these are not exclusive polarities like good and evil. There can be continuity between them, and there must be.”

Allowing children to develop immunity to contagious diseases “naturally,” without vaccination, is appealing to some of us. Much of that appeal depends on the belief that vaccines are inherently unnatural. But vaccines are of that liminal place between humans and nature—a mowed field, Berry might suggest, edged by woods. Vaccination is a kind of domestication of a wild thing, in that it involves our ability to harness a virus and break it like a horse, but its action depends on the natural response of the body to the effects of that once-wild thing.

The antibodies that generate immunity following vaccination are manufactured in the human body, not in factories. “In the pharmaceutical world,” the writer Jane Smith observes, “the great division is between biologicals and chemicals—drugs that are made from living substances and drugs that are made from chemical compounds.” Using ingredients from organisms, once living or still alive, vaccines invite the immune system to produce its own protection. The live viruses in vaccines are weakened, sometimes by having been passed through the bodies of animals, so that they cannot infect a healthy person. The most unnatural part of vaccination is that it does not, when all goes well, introduce disease or produce illness.

Infectious disease is one of the primary mechanisms of natural immunity. Whether we are sick or healthy, disease is always passing through our bodies. “Probably we’re diseased all the time,” as one biologist puts it, “but we’re hardly ever ill.” It is only when disease manifests itself as illness that we see it as unnatural, in the “contrary to the ordinary course of nature” sense of the word. When a child’s fingers blacken on his hand from Hib disease, when tetanus locks a child’s jaw and stiffens her body, when a baby barks for breath from pertussis, when a child’s legs are twisted and shrunken with polio—then disease does not seem natural.

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“I know you’re on my side,” an immunologist once remarked to me as we discussed the politics of vaccination. I did not agree with him, but only because I was uncomfortable with both sides, as I had seen them delineated. The debate over vaccination tends to be described with what the philosopher of science Donna Haraway would call “troubling dualisms.” These dualisms pit science against nature, public against private, truth against imagination, self against another, thought against emotion, and man against woman.

The metaphor of a “war” between mothers and doctors is sometimes used for conflicts over vaccination. Depending on who is employing the metaphor, the warring parties may be characterized as ignorant mothers and educated doctors, or intuitive mothers and intellectual doctors, or caring mothers and heartless doctors, or irrational mothers and rational doctors—sexist stereotypes abound.

Rather than imagine a war in which we are ultimately fighting against ourselves, perhaps we can accept a world in which we are all irrational rationalists. We are bound, in this world, to both nature and technology. We are all “cyborgs, hybrids, mosaics, chimeras,” as Haraway suggests in her feminist provocation A Cyborg Manifesto. She envisions a cyborg world “in which people are not afraid of their joint kinship with animals and machines, not afraid of permanently partial identities and contradictory standpoints.”

All of us who have been vaccinated are cyborgs, the cyborg scholar Chris Hables Gray suggests. Our bodies have been programmed to respond to disease, and modified by technologically altered viruses. As a cyborg and a nursing mother, I join my modified body to a breast pump, a modern mechanism to provide my child with the most primitive food. On my bicycle, I am part human and part machine, a collaboration that exposes me to injury. Our technology both extends and endangers us. Good or bad, it is part of us, and this is no more unnatural than it is natural.

When a friend asked, years ago, if my son’s birth was a “natural” birth, I was tempted to say that it was an animal birth. While his head was crowning, I was trying to use my own hands to pull apart my flesh and bring him out of my body. Or so I have been told, but I do not remember any intention to tear myself open—all I remember is the urgency of the moment. I was both human and animal then. Or I was neither, as I am now. “We have never been human,” Haraway suggests. And perhaps we have never been modern, either.

This article has been adapted from Eula Biss' On Immunity: An Inoculation.