According to the most recent census data, the uninsured portion of the United States population has fallen to 9 percent, with the sharpest drop registered among those living in households with incomes of less than 200 percent of the federal poverty level (which, translated into dollar terms, is the equivalent of an income of $48,600 a year for a family of four, or $23,760 for a single person). According to the Kaiser Family Foundation, more than 11 million people have purchased private health insurance plans through the Affordable Care Act exchanges, and a majority report incomes between 100 percent and 250 percent of the federal poverty level. It would seem that lower-income Americans are among the greatest beneficiaries of the A.C.A.’s reforms. And yet in some states this same population also remains, paradoxically, among the reforms’ greatest losers. This subpopulation is living inside a kind of “dead zone,” as Foy put it to me one day, searching for the right metaphor to describe her predicament. A long and suspended silence, she called it, “like when you can’t receive a single call, a single text.”

How these dead zones formed is a matter of unanticipated consequences. The A.C.A.’s architects did not predict that the Supreme Court would rule in 2012 that it was up to each state whether to expand Medicaid eligibility, which is how they imagined Americans with the most modest incomes would receive coverage. Even though the federal government would have helped fund the expansion, 19 states opted for ideological reasons not to do so, arguing that they are pushing back against government bloat and the fostering of dependency. A result was that the residents with the lowest incomes in those 19 states were now caught between two nonoptions: They made too much to qualify for Medicaid, or didn’t qualify at all, but they also made too little for publicly subsidized insurance on the exchanges, their income not high enough to trigger the refundable tax credits and cost-sharing that could make the possibility remotely affordable to someone making just a few dollars above the federal poverty level.

This paradox is referred to widely as the coverage gap. Most people in that gap are on the far side of middle age, with about one in eight edging toward 65 — a time in life when more serious health issues begin to emerge. Almost half are nonwhite. They are almost equally split along gender lines. About a quarter are supporting children, and everyone in the gap is more likely to be working (62 percent) than not (38 percent). Those with jobs work largely for small businesses that employ fewer than 50 people, which aren’t subject to A.C.A. penalties for not offering employer-based coverage. Most people in the gap who have jobs work full time. They are agricultural workers, primarily, or service-industry employees, but some hold jobs in education, health and social services, professional administration or manufacturing.

There is little disagreement in the existing literature about the negative effects of being uninsured: You are more likely to receive a diagnosis of late-stage cancer; you are more likely to postpone or forgo care, resulting in more severe consequences as treatable illnesses become increasingly complicated with delay. There’s also good, hard evidence from large-scale studies on “the wear and tear that worry and stress has on people who don’t have insurance coverage,” says Genevieve Kenney, co-director of the Health Policy Center at the Urban Institute, noting one of the most compelling and most cited: a study in Oregon that found that offering Medicaid to the uninsured reduced bad medical debts, decreased the likelihood of choosing to cover medical expenses over other bills, buffered them from catastrophic out-of-pocket payments and significantly reduced depression.

And yet the coverage gap is a new enough phenomenon that scant on-the-ground research exists into the particular and distinct ways that it is playing out in people’s day-to-day lives. Already, a scattering of researchers, mostly medical anthropologists, have taken steps to follow communities where significant numbers of people are caught in the coverage gap, in order to gain the kinds of insights “you don’t get from a single snapshot, a one-off survey,” says Heide Castañeda, an associate professor in the anthropology department at the University of South Florida. Those insights reveal, as she puts it, “not just how vulnerable people are, but how much agency they have, how much initiative they have to try and find a solution when none seems to exist. You can’t code that in a binary way; you have to watch it unfold over time.”

In Castañeda’s case, she has been studying the lives of the uninsured in Hidalgo County, Tex., which has one of the highest rates of uninsurance in the nation. She has traced the health of dozens of families over the last four years, simultaneous to the rollout of the A.C.A., documenting how they have responded to illness or chronic disease or accident. What she has seen is that to live in the gap demands a creative, improvisational mode of survival — one that often masks the true extent of the disparities to anyone on the outside. “It might be true no one is dying in the streets,” she says. “But the uninsured are dying younger; people’s life expectancy is affected, people’s ability to work is affected. These informal types of health care, as important as they are, actually help us not to see that.”