The smallpox virus hopscotched across the post-Civil War South, invading the makeshift camps where many thousands of newly freed African-Americans had taken refuge but leaving surrounding white communities comparatively unscathed. This pattern of affliction was no mystery: In the late 1860s, doctors had yet to discover viruses, but they knew that poor nutrition made people more susceptible to illness and that poor sanitation contributed to the spread of disease. They also knew that quarantine and vaccination could stop an outbreak in its tracks; they had used those very tools to prevent a smallpox outbreak from ravaging the Union Army.

Smallpox was not the only health disparity facing the newly emancipated, who at the close of the Civil War faced a considerably higher mortality rate than that of whites. Despite their urgent pleas for assistance, white leaders were deeply ambivalent about intervening. They worried about black epidemics spilling into their own communities and wanted the formerly enslaved to be healthy enough to return to plantation work. But they also feared that free and healthy African-Americans would upend the racial hierarchy, the historian Jim Downs writes in his 2012 book, “Sick From Freedom.”

Federal policy, he notes, reflected white ambivalence at every turn. Congress established the medical division of the Freedmen’s Bureau — the nation’s first federal health care program — to address the health crisis, but officials deployed just 120 or so doctors across the war-torn South, then ignored those doctors’ pleas for personnel and equipment. They erected more than 40 hospitals but prematurely shuttered most of them.

White legislators argued that free assistance of any kind would breed dependence and that when it came to black infirmity, hard labor was a better salve than white medicine. As the death toll rose, they developed a new theory: Blacks were so ill suited to freedom that the entire race was going extinct. “No charitable black scheme can wash out the color of the Negro, change his inferior nature or save him from his inevitable fate,” an Ohio congressman said.

One of the most eloquent rejoinders to the theory of black extinction came from Rebecca Lee Crumpler, the nation’s first black female doctor. Crumpler was born free and trained and practiced in Boston. At the close of the war, she joined the Freedmen’s Bureau and worked in the freed people’s communities of Virginia. In 1883, she published one of the first treatises on the burden of disease in black communities. “They seem to forget there is a cause for every ailment,” she wrote. “And that it may be in their power to remove it.”

In the decades following Reconstruction, the former slave states came to wield enormous congressional power through a voting bloc that was uniformly segregationist and overwhelmingly Democratic. That bloc preserved the nation’s racial stratification by securing local control of federal programs under a mantra of “states’ rights” and, in some cases, by adding qualifications directly to federal laws with discriminatory intent.

The African-American ward of the Jackson Memorial Hospital in Miami in 1944. Jerry Cooke/The Life Images Collection, via Getty Images

As the Columbia University historian Ira Katznelson and others have documented, it was largely at the behest of Southern Democrats that farm and domestic workers — more than half the nation’s black work force at the time — were excluded from New Deal policies, including the Social Security and Wagner Acts of 1935 (the Wagner Act ensured the right of workers to collective bargaining), and the Fair Labor Standards Act of 1938, which set a minimum wage and established the eight-hour workday. The same voting bloc ensured states controlled crucial programs like Aid to Dependent Children and the 1944 Servicemen’s Readjustment Act, better known as the G.I. Bill, allowing state leaders to effectively exclude black people.

[Myths about physical racial differences were used to justify slavery — and are still believed by doctors today.]

In 1945, when President Truman called on Congress to expand the nation’s hospital system as part of a larger health care plan, Southern Democrats obtained key concessions that shaped the American medical landscape for decades to come. The Hill-Burton Act provided federal grants for hospital construction to communities in need, giving funding priority to rural areas (many of them in the South). But it also ensured that states controlled the disbursement of funds and could segregate resulting facilities.

Professional societies like the American Medical Association barred black doctors; medical schools excluded black students, and most hospitals and health clinics segregated black patients. Federal health care policy was designed, both implicitly and explicitly, to exclude black Americans. As a result, they faced an array of inequities — including statistically shorter, sicker lives than their white counterparts. What’s more, access to good medical care was predicated on a system of employer-based insurance that was inherently difficult for black Americans to get. “They were denied most of the jobs that offered coverage,” says David Barton Smith, an emeritus historian of health care policy at Temple University. “And even when some of them got health insurance, as the Pullman porters did, they couldn’t make use of white facilities.”

In the shadows of this exclusion, black communities created their own health systems. Lay black women began a national community health care movement that included fund-raising for black health facilities; campaigns to educate black communities about nutrition, sanitation and disease prevention; and programs like National Negro Health Week that drew national attention to racial health disparities. Black doctors and nurses — most of them trained at one of two black medical colleges, Meharry and Howard — established their own professional organizations and began a concerted war against medical apartheid. By the 1950s, they were pushing for a federal health care system for all citizens.

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That fight put the National Medical Association (the leading black medical society) into direct conflict with the A.M.A., which was opposed to any nationalized health plan. In the late 1930s and the 1940s, the group helped defeat two such proposals with a vitriolic campaign that informs present-day debates: They called the idea socialist and un-American and warned of government intervention in the doctor-patient relationship. The group used the same arguments in the mid-’60s, when proponents of national health insurance introduced Medicare. This time, the N.M.A. developed a countermessage: Health care was a basic human right.

Medicare and Medicaid were part of a broader plan that finally brought the legal segregation of hospitals to an end: The 1964 Civil Rights Act outlawed segregation for any entity receiving federal funds, and the new health care programs soon placed every hospital in the country in that category. But they still excluded millions of Americans. Those who did not fit into specific age, employment or income groups had little to no access to health care.

In 2010, the Affordable Care Act brought health insurance to nearly 20 million previously uninsured adults. The biggest beneficiaries of this boon were people of color, many of whom obtained coverage through the law’s Medicaid expansion. That coverage contributed to a measurable decrease in some racial health disparities, but the success was neither as enduring nor as widespread as it might have been. Several states, most of them in the former Confederacy, refused to participate in Medicaid expansion. And several are still trying to make access to the program contingent on onerous new work requirements. The results of both policies have been unequivocal. States that expanded Medicaid saw a drop in disease-related deaths, according to the National Bureau of Economic Research. But in Arkansas, the first state to implement work requirements, nearly 20,000 people were forced off the insurance plan.

One hundred and fifty years after the freed people of the South first petitioned the government for basic medical care, the United States remains the only high-income country in the world where such care is not guaranteed to every citizen. In the United States, racial health disparities have proved as foundational as democracy itself. “There has never been any period in American history where the health of blacks was equal to that of whites,” Evelynn Hammonds, a historian of science at Harvard University, says. “Disparity is built into the system.” Medicare, Medicaid and the Affordable Care Act have helped shrink those disparities. But no federal health policy yet has eradicated them.