My son is fully vaccinated, but there is one immunization on the standard schedule that he did not receive on time. This was meant to be his very first shot, the hep B administered to most babies immediately after birth. In the months before my son was born, while I was teaching at the university and hauling a used crib through the snow and moving bookshelves to make room for the crib, I began spending my evenings reading articles about immunization. I was already aware, before I became pregnant, of some fears around vaccination. But I was not prepared for the labyrinthine network of interlocking anxieties I would discover during my pregnancy, the proliferation of hypotheses, the minutiae of additives, the diversity of ideologies.

Finding that the reach of my subject had far exceeded the limits of my late-night research by the time my baby was due, I visited the pediatrician I had chosen to be my son’s doctor. A number of friends had offered his name when I asked for a recommendation, and so had my midwife, who referred to him as “left of center.” When I asked the pediatrician what the purpose of the hep B vaccine was, he answered, “That’s a very good question,” in a tone that I understood to mean this was a question he relished answering. Hep B was a vaccine for the inner city, he told me, designed to protect the babies of drug addicts and prostitutes. It was not something, he assured me, that people like me needed to worry about.

All that this doctor knew of me then was what he could see. He assumed, correctly, that I did not live in the inner city. It did not occur to me to clarify for the doctor that although I live in the outer city of Chicago, my neighborhood is very much like what some people mean when they use the term inner city. In retrospect, I am ashamed by how little of his racial code I registered. Relieved to be told that this vaccine was not for people like me, I failed to consider what exactly that meant.

The belief that public health measures are not intended for people like us is widely held by many people like me. Public health, we assume, is for people with less—less education, less- healthy habits, less access to quality health care, less time and money. I have heard mothers of my class suggest, for instance, that the standard childhood immunization schedule groups together multiple shots because poor mothers will not visit the doctor frequently enough to get the twenty-six recommended shots separately. No matter that any mother, myself included, might find so many visits daunting. That, we seem to be saying of the standard schedule, is for people like them.

In an article for Mothering magazine, the journalist Jennifer Margulis expresses outrage that newborn infants are routinely vaccinated against hep B and wonders why she was encouraged to vaccinate her daughter “against a sexually transmitted disease she had no chance of catching.” Hep B is transmitted not only by sex, but through bodily fluids, so the most common way that infants contract hep B is from their mothers. Babies born to women who are infected with hep B—and mothers can carry the virus without their knowledge—will almost certainly be infected if they are not vaccinated within twelve hours of birth. The virus can also be passed through close contact between children, and people of any age can carry it without symptoms. Like human papillomavirus and a number of other viruses, hep B is a carcinogen, and it is most likely to cause cancer in people who contract it when they are young.

One of the mysteries of hep B immunization is that vaccinating only “high risk” groups, which was the original public health strategy, did not bring down rates of infection. When the vaccine was introduced in 1981, it was recommended for prisoners, health care workers, gay men, and IV drug users. But rates of hep B infection remained unchanged until the vaccine was recommended for all newborns a decade later. Only mass vaccination brought down the rates of infection, and it has now virtually eliminated the disease in children.

The concept of a “risk group,” Susan Sontag writes, “revives the archaic idea of a tainted community that illness has judged.” Risk, in the case of hep B, turns out to be a rather complicated assessment. There is risk in having sex with just one partner, or traveling through the birth canal. In many cases, the source of infection is never known. I decided, before I knew how much blood I would lose in childbirth, that I did not want my son to be vaccinated against hep B. I did not belong to a risk group at the moment he was born, but by the time I put him to my breast I had received a blood transfusion and my status had changed.

When the last nationwide smallpox epidemic began in 1898, some people believed that whites were not susceptible to the disease. It was called “Nigger itch,” or, where it was associated with immigrants, “Italian itch” or “Mexican bump.” When smallpox broke out in New York City, police officers were sent to help enforce the vaccination of Italian and Irish immigrants in the tenements. And when smallpox arrived in Middlesboro, Kentucky, everyone in the black section of town who resisted vaccination was vaccinated at gunpoint. These campaigns did limit the spread of the disease, but all the risk of vaccination, which at that time could lead to infection with tetanus and other diseases, was absorbed by the most vulnerable groups. The poor were enlisted in the protection of the privileged.

Debates over vaccination, then as now, are often cast as debates over the integrity of science, though they could just as easily be understood as conversations about power. The working-class people who resisted Britain’s 1853 provision for free, mandatory vaccination were concerned, in part, with their own freedom. Faced with fines, imprisonment, and the seizure of their property if they did not vaccinate their infants, they sometimes com- pared their predicament to slavery.

Vaccination, like slavery, raises some pressing questions about one’s rights to one’s own body. But as the historian Nadja Durbach has noted, antivaccinators were often more interested in abolition as a metaphor for individual liberty than they were in the cause as a shared purpose. It was not in the recklessly selfless spirit of John Brown, who was hanged with his sons for their doomed effort to free slaves, that white workers resisted vaccination. “Anti-vaccinators were quick to draw on the political, emotive, or rhetorical value of the slave, or of the colonized African,” Durbach writes of the movement in Britain. “They were quicker still to claim that the suffering of white English citizens took precedence over that of the op- pressed elsewhere.” Their primary concern, in other words, was with people like them.

In her history of that movement, Durbach returns often to the idea that vaccine resisters saw their bodies “not as potentially contagious and thus dangerous to the social body, but as highly vulnerable to contamination and violation.” Their bodies were, of course, both contagious and vulnerable. But in a time and place where the bodies of the poor were seen as a liability to public health, as dangerous to others, it fell to the poor to articulate their vulnerability.

If it was meaningful then for the poor to assert that they were not purely dangerous, I suspect it might be just as meaningful now for the rest of us to accept that we are not purely vulnerable. The middle class may be “threatened,” but we are still, just by virtue of having bodies, dangerous. Even the little bodies of children, which our time encourages us to imagine as absolutely vulnerable, are dangerous in their ability to spread disease. Think of the unvaccinated boy in San Diego, for in- stance, who returned from a trip to Switzerland in 2008 with a case of measles that infected his two siblings, five schoolmates, and four children in his doctor’s waiting room. Three of those children were infants too young to be vaccinated, and one had to be hospitalized.

Unvaccinated children, a 2004 analysis of CDC data reveals, are more likely to be white, to have an older married mother with a college education, and to live in a household with an income of $75,000 or more—like my child. Unvaccinated children also tend to be clustered in the same areas, raising the probability that they will contract a disease that can then be passed, once it is in circulation, to undervaccinated children. Undervaccinated children, meaning children who have received some but not all of their recommended immunizations, are more likely to be black, to have a younger unmarried mother, to have moved across state lines, and to live in poverty.

“Vaccination works,” my father explains, “by enlisting a majority in the protection of a minority.” He means the minority of the population that is particularly vulnerable to a given disease. The elderly, in the case of influenza. Newborns, in the case of pertussis. Pregnant women, in the case of rubella. But when relatively wealthy white women vaccinate our children, we may also be participating in the protection of some poor black children whose single mothers have recently moved and have not, as a product of circumstance rather than choice, fully vaccinated them. This is a radical inversion of the historical application of vaccination, which was once just another form of bodily servitude extracted from the poor for the benefit of the privileged. There is some truth, now, to the idea that public health is not strictly for people like me, but it is through us, literally through our bodies, that certain public health measures are enacted.

The concept of clear and present danger was once used to defend mandatory vaccination in times of epidemic. And the term conscientious objector, now associated primarily with war, originally referred to those who refused vaccination. Britain’s Compulsory Vaccination Act of 1853 required the vaccination of all infants, and was widely resisted. After later legislation allowed for resisters to be repeatedly fined, those who could not pay had their belongings seized and auctioned, or were imprisoned. In 1898, the government added a conscience clause to the act, allowing parents to apply for exemption. The clause was rather vague, requiring only that the objector “satisfy” a magistrate that her objection was a matter of conscience. This would result in thousands of cases of conscientious objection, in some places accounting for the majority of all births, as well as a debate over what exactly it meant to possess a conscience.

Before the term conscientious objector was written into law, it was used by vaccine resisters to distinguish themselves from negligent parents who had not bothered to vaccinate their children. The word conscientious was meant to signal that this was an intentional decision made by caring parents. Conscientious objectors argued that a conscience could not and should not be evaluated, and the magistrates themselves were vexed by the problem of whether or not to demand some sort of evidence to support a claim to conscience. “I don’t understand the Act,” one magistrate said in frustration. “I have seen you, and you have told me you have a conscientious objection; I don’t know whether that is enough.” The word satisfy was eventually removed from the conscience clause and a series of memoranda specified that an objector must hold an “honest” belief that vaccination would harm her child but that her belief need not be “reasonably founded.” In debating the law, parliamentarians determined that the conscience was very difficult to define.

From the advent of the conscience clause until now, the Oxford English Dictionary has consistently defined conscience primarily in terms of right and wrong. “The sense of right and wrong as regards things for which one is responsible” now appears in its first definition. The next six definitions mention ethical values, justice, equity, correct judgments, scruple, knowledge, insight, and God, with feelings and heart entering into the eighth and ninth definitions, along with the notations “now rare” and “obsolete.”

George Washington, a survivor of smallpox, wrestled with the question of whether or not to require inoculation for revolutionary soldiers long before vaccination became a question of conscience. In 1775, roughly a third of the Continental Army fell ill to smallpox while laying siege to Quebec. They were eventually forced to retreat in the first battlefield defeat of this country’s history. The deadliest smallpox epidemic the colonies had seen was in the process of taking 100,000 lives, but small- pox was endemic in England and most of the British soldiers were immune, having survived it as children. This was before the invention of vaccination, and Washington was reluctant to subject his troops to variolation, which had known dangers and was illegal in some of the colonies. Several times, he ordered inoculation and then withdrew the order days later. Finally, with rumors in the air of a British plan to spread smallpox as a form of biological warfare, Washington definitively ordered the inoculation of all new recruits.

If we owe the existence of this nation in some part to compulsory inoculation, we also owe some of its present character to the resistance against compulsory vaccination. Early vaccine refusers were among the first to make legal challenges to the growing reach of police power in the United States. We have them to thank for the fact that we can no longer be vaccinated at gunpoint, and perhaps also for the fact that women cannot be denied abortions. Some pivotal reproductive rights cases in the 1970s cited as their precedent Jacobson v. Massachusetts, a 1905 Supreme Court case in which a minister defended his refusal of vaccination on the grounds that a previous vaccination had damaged his health. But this case has also been used as a precedent for defending warrantless searches and the detainment of US citizens. The ruling in Jacobson was an effort to balance the interests of the collective and the power of the state against the rights of the individual. It upheld compulsory vaccination laws, but required that states offer exemptions for individuals who might be subject to injustice and oppression under those laws.

The United States has never had a federal compulsory vaccination law. In the early twentieth century some states had compulsory laws, but two-thirds of the states did not, and a couple states had laws against compulsion. In some school districts, children were—as they are now—required to be vaccinated in order to attend public school, but this requirement was often loosely enforced. A third of the school children in Greenville, Pennsylvania, for example, were granted medical exemptions from vaccination.

The only vaccine routinely recommended at that time was the smallpox vaccine, which had serious side effects and was frequently contaminated with bacteria. A new, milder strain of smallpox appeared in this country around the turn of the century and Variola minor, as it is now known, was killing only around 1% of the people who contracted it, as opposed to the 30% who typically died from Variola major. With smallpox taking fewer lives, unorganized opposition to vaccination became an antivaccination movement led by activists like Lora Little, who offered empowering advice: “Be your own doctor. Run your own machine.” In some places, armed mobs drove vaccinators away. “Vaccination riots,” the journalist Arthur Allen writes, “were not at all uncommon.”

Long before the term immunity was used in the context of disease, it was used in the context of law to describe an exemption from service or duty to the state. Immunity came to mean freedom from disease as well as freedom from service in the late nineteenth century, after states began requiring vaccination. In a peculiar collision of meanings, the exemption from immunity made possible by the conscience clause was a kind of immunity in itself. And allowing oneself to remain vulnerable to disease remains a legal privilege today.

Dictionaries aside, what it means to have a conscience may be no more clear to us now than it was in 1898. We do recognize when it is lacking — she has no conscience, we say. But what exactly is missing? I put this question to my sister, who teaches ethics at a Jesuit college and is a member of the North American Kant Society. “It’s tricky,” she says. “In the eighteenth century, Kant wrote that we have a duty to ourselves to examine our conscience. This implies that it’s not transparent, that it must be scrutinized and deciphered. Kant thought of conscience as an inner judge and used the metaphor of a courtroom to explain its operation. In the courtroom of conscience, the self is both judge and judged.”

I ask her if this means our conscience emerges from thought and is a product of our minds. “It’s an evolving concept,” she says. “It may have once been more closely associated with the emotions, but we still say we feel a pang of conscience—it involves a unity of thought and feeling.” Kant, she tells me, called the inner judge a “scrutinizer of hearts.”

“The part that’s tricky,” my sister says, “is how you discern between a sense of discomfort and what your conscience is telling you.” This question remains with me, and I am disturbed by the possibility that I could mistake the call of my conscience for something else. I ask a former professor of mine, a novelist who teaches the Old Testament as literature, how one recognizes one’s own conscience. She looks at me sternly and says, “It’s a very distinct feeling. I don’t think one’s conscience is easily confused with any other feeling.”

“Morality can’t be fully private,” my sister tells me, “for many of the same reasons that a language can’t be fully private. You can’t be intelligible only to yourself. But thinking of the conscience as a private sense of right and wrong suggests that our collective understandings of justice can be insufficient. An individual might resist flaws in the dominant moral code and thus create the possibility for reform — there are all sorts of historical examples of this. But another way to think about the conscience is as an inner voice that keeps your actions in line with publicly defendable moral standards. It reforms you.”

One of the mercies of immunity produced by vaccination is that a small number of people can forgo vaccination without putting themselves or others at greatly increased risk. But the exact number of people this might be — the threshold at which herd immunity is lost and the risk of disease rises dramatically for both the vaccinated and the unvaccinated — varies depending on the disease and the vaccine and the population in question. We know the threshold, in many cases, only after we’ve exceeded it. And so this puts the conscientious objector in the precarious position of potentially contributing to an epidemic. Here we may suffer what economists call moral hazard, a tendency to take unwise risks when we are protected by insurance. Our laws allow for some people to exempt themselves from vaccination, for reasons medical or religious or philosophical. But deciding for ourselves whether we ought to be among that number is indeed a matter of conscience.

In a section of The Vaccine Book titled “Is it your social responsibility to vaccinate your kids?” Dr. Bob asks, “Can we fault parents for putting their own child’s health ahead of that of the kids around him?” This is meant to be a rhetorical question, but Dr. Bob’s implied answer is not mine. In another section of the book, Dr. Bob writes of his advice to parents who fear the MMR vaccine, “I also warn them not to share their fears with their neighbors, because if too many people avoid the MMR, we’ll likely see the disease increase significantly.”

I do not need to consult an ethicist to determine that there is something wrong there, but my sister clarifies my discomfort. “The problem is in making a special exemption just for yourself,” she says. This reminds her of a way of thinking proposed by the philosopher John Rawls: Imagine that you do not know what position you are going to hold in society — rich, poor, educated, insured, no access to health care, infant, adult, HIV positive, healthy immune system, etc. — but that you are aware of the full range of possibilities. What you would want in that situation is a policy that is going to be equally just no matter what position you end up in.

“Consider relationships of dependence,” my sister suggests. “You don’t own your body — that’s not what we are, our bodies aren’t independent. The health of our bodies always depends on choices other people are making.” She falters for a moment here, and is at a loss for words, which is rare for her. “I don’t even know how to talk about this,” she says. “The point is there’s an illusion of independence.”

This post is excerpted from On Immunity: An Inoculation copyright © 2014 by Eula Biss. It was printed by permission of Graywolf Press.