Brittny Giles, of Flint, Mich., gives her 9-month-old daughter, Joel, a kiss before putting her to bed. Giles recently had her children's blood lead levels tested, which came back low, but she said she is still concerned. (Brittany Greeson/For The Washington Post)

Steven Mintz is a history professor at the University of Texas at Austin and the author of 14 books, including “Huck’s Raft: A History of American Childhood” and “The Prime of Life: A History of Modern Adulthood.”

The news reports of children poisoned by tap water in Flint, Mich., and of babies’ brains apparently damaged by Zika in Brazil are horrifying for some of the same reasons. In both cases, the threat is invisible. Lead has been leaching into Flint’s water from pipes buried deep underground. The Zika virus, carried by tiny mosquitoes, produces no symptoms in 80 percent of infected people. With Zika and lead, it can take months or years to realize that irreversible damage has been done — for microcephaly to become evident in a fetus, for lead poisoning to show up as learning disabilities and behavior problems. In the meantime, it’s difficult to trust what health authorities are saying, because government officials in Flint seem to have acted with negligence verging on malice and because doctors are still trying to understand Zika.

But perhaps the most chilling parallel between Zika and lead is that they both seem to assault children. In an era of heightened anxiety about protecting kids from contamination and harm, they play on our greatest fears.

What will we do with those fears? Typically, we respond in one of two unhelpful ways. We overestimate the risk and focus exclusively on safeguarding our own children. Or we discount the danger, telling ourselves it can’t happen to our kids. We ignore what disease pathologies and water-distribution systems should remind us: how connected we are.

Rep. Elijah Cummings offered an impassioned ode to children at this month’s congressional hearing on Flint. “I’ve often said that our children are the living messages we send to a future we will never see,” the Maryland Democrat said. “The question is: What will they leave us? And how will we send them into that future? Will we send them strong? Will we send them hopeful? Will we rob them of their destiny? Will we rob them of their dreams? No, we will not do that!”

This is a fairly modern way of thinking about children — as vulnerable beings we should cultivate and protect. Of course, the impulse to protect children may have an evolutionary dimension. Individual parents may be genetically programmed to invest in the survival of their offspring. But the notion that children are innocent, fragile creatures that society as a whole should shelter from contamination is a relatively recent idea.

In my research, I’ve documented how colonial Americans considered children depraved. Babies, they thought, were dangerously unformed, immoral and animalistic, as evidenced by their inability to speak or stand. Parents were expected to teach their children to walk upright, help them to memorize scripture and then put them to work as soon as possible. By contemporary standards, there was a surprising lack of concern for children’s welfare. Children frequently suffered severe burns playing near fireplaces, fell into uncovered wells or were attacked by wild animals, all suggesting an absence of adult supervision. Yes, many children died of diseases not then treatable. But they also died of easily preventable causes.

Around the middle of the 18th century, attitudes began to shift. Drawing on John Locke’s notion that children are blank slates and Jean-Jacques Rousseau’s concept of children as pure and spontaneous, a romantic vision of childhood encouraged parents to shelter their children from adult realities to preserve their innocence. Parents kept children at home, in school and away from work longer than in the past. And an array of new institutions — from public schools to orphanages to children’s hospitals — emerged to protect children’s welfare.

Still, the overwhelming majority of 19th-century American autobiographies report children being disciplined with a cane, paddle or leather strap. Child abuse and neglect became criminal offenses starting only in the 1870s. Mandatory schooling did not become universal until the early 20th century. The United States abolished child labor only in 1938.

Slowly, incrementally, protecting children from harm became a public priority. Indeed, the growth of the American welfare state, from Aid to Dependent Children in the 1930s to Head Start, established in 1965, was largely driven by concern for children’s well-being. This effort to ensure a “right to childhood,” free from abuse or exploitation, was bipartisan and reflected a growing consensus that children, by virtue of their special vulnerability and their role as our collective future, deserve public support.

Unfortunately, in recent decades, collective concern has often given way to something more inward looking. Anxiety about children’s welfare is at the heart of modern parenting. As soon as pregnancy is confirmed, many parents start to worry about threats to their developing fetus from various foods, environmental toxins and maternal stress. A child’s first year brings apprehension about sudden infant death syndrome, choking, sharp corners and electrical sockets. Even as children grow and gain independence, parents continue to agonize about their physical health, personality development, psychological well-being and academic performance.

As a result, more moms and dads drive their kids to school, producing congestion in school zones that actually endangers children. Parents discourage their children from roaming freely outdoors, which contributes to obesity and excessive time spent in front of screens. Often, instead of translating their fears into a communal response — for example, by increasing supervision at parks and playgrounds — parents react to their worries on an individual basis, to the detriment of all children’s well-being.

With Zika and with lead poisoning, children seem to be especially vulnerable. Scientists think their developing brains transform what’s mild in adults into serious conditions.

In the context of generalized anxiety about kids, it’s easy to see how panic can result from World Health Organization warnings about the Zika virus “spreading explosively” and newspaper headlines such as “Untold cities across America have higher rates of lead poisoning than Flint.”

Across the United States, pregnant women are calling their doctors with questions about Zika. “It’s very terrifying,” a pregnant woman in Dallas told her local NBC affiliate. Her doctor, Sheila Chhutani, reminded her that there are no mosquitoes carrying Zika in northern Texas — or anywhere in the United States, for that matter. “I’m not worried about my pregnant patients here,” Chhutani said. “I’m still more worried about them getting the flu, or making sure they get their flu shots, more than I am about the Zika virus.”

At the same time, parents are questioning whether they can trust the water coming out of their taps or what government officials say about it. Pediatricians in New Haven, Conn., for example, tried to assure their patients that the water was safe — but then privately wondered whether or not that was true.

The anxious parents may feel genuine anguish for the children and families who have actually been affected by Zika or lead poisoning. But so far, for the most part, their responses have been self-regarding. They want to do everything they can to advance their own children’s life prospects.

What happens from here?

Parents may overestimate the risks and go to great lengths to protect their own children. We saw this in the 1970s and 1980s, when exaggerated fears of stranger abductions led many parents to severely restrict their children’s outdoor play, while alarm over purported sexual abuse in day-care centers led to prosecutions that were ultimately overturned. More recently, a significant minority of parents have refused to vaccinate their children out of a misplaced fear that this might contribute to autism. And even though school shootings are rare, many parents say they fear for their children when they drop them off each day.

In other situations, parents may compartmentalize harm, dismissing it as something that happens to other people’s children — to people who are somehow less deserving, who have bad luck or who live dramatically different lives.

Two times now, heartbreaking images of Syrian children have provoked passions in the United States. In September 2013, there were the videos of children writhing in pain, dying, after a chemical weapons attack allegedly by President Bashar al-Assad’s forces. And last September there was the Syrian toddler whose body washed up on a Turkish beach after a refu­gee boat capsized. Yet because these children were far away, because their experience was so unlike ours, passion didn’t move us to action. President Obama’s proposal for airstrikes against the Syrian regime proved unpopular. Assad remains in power. And while some Syrian refugees have been welcomed into the United States, they have also been confronted by calls to close the borders and ban Muslims.

Similarly, the thousands of Central American children seeking to escape violence and poverty by coming to the United States captured the public’s attention in the summer of 2014. But pleas for compassion competed with anti-immigrant sentiment. And now, outrage over deportation raids targeting those children and their families is largely limited to immigration rights advocates and Latino leaders.

Compartmentalizing is easier for people to do when the children are from another country and speak a different language. It also happens when the threat disproportionately affects poor children, as with Brazil’s Zika and Flint’s water.

But a communal response is critical in those cases, because poor people don’t have the resources to protect their children like wealthier parents can. They can’t hunker down in their air-conditioned homes until the mosquito threat is under control or a vaccine is developed. They may not have access to contraceptives, prenatal care or abortion. They don’t have the luxury of forgoing straight tap water in favor of bottled water and reverse-osmosis filters. They often can’t make their voices heard.

We can help children by pressing our government to provide communities with resources to replace decaying water systems, to conduct trustworthy lead-level monitoring, and to offer health and educational supports to those poisoned in the past. We can help if the United States is willing to support mosquito control in Zika-infected regions, to improve and expand testing, and to accelerate the quest for a vaccine.

The key question is whether Americans will respond to Zika and lead poisoning as problems predominantly of the poor, met with pity and a shrug of the shoulders, or as a call to arms demanding concerted collective action and a deepening commitment to children’s welfare, irrespective of nationality or class.

Will we fulfill our responsibility to children beyond our own?

Twitter: @Steven_Mintz