In the words of the immortal pioneer of soul music, Sam Cooke, “it’s been a long, a long time coming, but I know a change gonna come.” After nearly a century of on- and off-again attempts, the United States has comprehensive health-care reform.

The Affordable Care Act does not contain all the provisions that some of us hoped for. Still, it offers enormous possibility to improve both individual and public health. As the United States welcomes more than 25,000 delegates from across the world to the International AIDS Conference, we have something new and important to tell our colleagues. The U.S. Supreme Court’s decision mostly upholding the ACA bears the promise of bringing the United States closer to the many countries around the world that guarantee their residents access to health care.

For many people living with HIV, the ACA will greatly improve and prolong their lives. Put simply, the current health insurance system in the United States does not work for many people living with HIV. While most working-age Americans have private health insurance, most people living with HIV do not. This is largely because most private insurance comes through employers, and many people living with HIV are unemployed. Of those who work, many are in low-wage jobs that are less likely to offer medical coverage. Purchasing insurance in the individual market is also not an option for most people living with HIV — they will be denied for having a pre-existing condition, or policies will be prohibitively expensive. All of these factors contribute to the fact that a disproportionately high number of HIV-positive U.S. residents are uninsured. They join approximately 50 million other U.S. residents who live without health insurance.

The ACA has created plans that allow people with pre-existing medical conditions, including HIV, to buy health insurance. Starting in 2014, insurance companies may not refuse to cover people with pre-existing conditions, and they may not discriminate based on health status or gender. Insurance companies will be barred from imposing a dollar limit on people’s benefits. For people with complex, chronic illnesses, such as HIV, this will help ensure that the insurance they’ve paid for will be there when they need it.

Starting in 2014, small businesses and low- to moderate-income individuals without employer-sponsored health insurance will be able to purchase private coverage through state health-insurance exchanges. People with income up to 400 percent of the federal poverty level (up to about $45,000 for an individual and $92,000 for a family of four) will receive federal tax credits and subsidies designed to make insurance more affordable.

The ACA also creates the most significant change to Medicaid since the program’s inception in 1965 — one that will have the biggest impact on HIV-positive people. Medicaid is already the largest source of health insurance for people living with HIV. But Medicaid as it exists now is not a health-care system — it is a disability-care system. Contrary to what many people believe, being poor is not currently enough to qualify for Medicaid. A person must be not only low-income but also fit a particular eligibility category, such as being a child, being pregnant or being disabled. In the context of HIV, this means that an HIV-positive person must become disabled by AIDS, such as by developing opportunistic infections like lymphoma or encephalopathy, before becoming eligible for the medical care that could have prevented the disease from progressing in the first place. This cruel Catch-22 is neither a compassionate nor an economically sensible approach.

Under the ACA, states are required to expand Medicaid eligibility in 2014 to most individuals with income up to 133 percent of the federal poverty level (about $15,000 for an individual or $31,000 for a household of four), with the notable exception of legal immigrants in their first five years in the United States and all undocumented immigrants. This will make Medicaid, for the first time, a health-care program for low-income people.

But worrisome challenges remain. The Supreme Court also ruled that the federal government cannot withhold all Medicaid funding from states that do not comply. Only new funding for expanded Medicaid may be withheld; noncompliant states will retain funding for their existing Medicaid programs. In effect, this makes Medicaid expansion optional for states. Despite the fact that the federal government will pay for the overwhelming majority of the ACA’s Medicaid expansion costs, some governors, including those in Texas and Florida, have indicated publicly that they will not expand eligibility.

Governors have cited the cost and a states’ rights rationale as reasons to oppose Medicaid expansion. It is also likely that opposition comes from political antipathy to implementing a major Obama administration initiative or from philosophical objections to providing low-income people access to a program viewed as “welfare.” This raises the possibility that many of our lowest-income Americans — particularly those living in states that have historically had the most restrictive Medicaid programs — will continue to be denied access to medical coverage. This is an unacceptable outcome. It undermines efforts to reduce health disparities and the ACA goal of ensuring that the United States finally joins many countries around the world in providing nationwide access to health care for poor residents.

Those of us who have spent our careers working to improve access to health care have waited a long, long time for the positive changes the ACA offers. The Affordable Care Act holds the promise that, one day, health care may be considered a human right in the United States, as it is in much of the rest of the world. For this promise to be fulfilled, however, we must ensure that the ACA is fully implemented and that our poorest and most vulnerable — including many living with HIV — are not left out.

Greenwald is the director of the Center for Health Law and Policy Innovation at Harvard Law School, and Rosenberg is the center’s associate director.