Last month, President Trump signed into law the First Step Act, bipartisan legislation aimed at reforming the criminal justice system. Among other reforms, the Act increases the number of “good time” credits inmates can earn and apply towards a reduced sentence, modifies the “three strikes” rule so people with three or more convictions are automatically given 25 years in prison instead of life, and makes it easier for judges to opt out of delivering mandatory minimum sentences.

While the reach of these reforms is relatively modest, affecting only federal prison inmates—nearly 180,000 of the total incarcerated population of about 2.2 million—the Act is nevertheless a step in the right direction. It points towards a justice system that is more what it was always supposed to be: just. It also points towards better health. Mass incarceration is a significant public health challenge in the US.

Inmates face a range of harmful exposures, from hepatitis to HIV, a risk driven by the prison environment. Incarceration also undermines the health of families. Close to 2.7 million children have an incarcerated parent. More than half of these parents were the primary earners supporting their kids. Their absence can make it harder for children to grow up with the basic necessities needed for a healthy life. More broadly, mass incarceration can threaten the health of whole communities. People living in neighborhoods with a high incarceration rate are likelier to meet the criteria for major depressive disorder. There are also abundant data that suggest a link between incarceration and rates of violence in communities.

This poor health springs from the fundamental injustice that drives mass incarceration: the exorbitantly high sentences that apply to nonviolent drug offenses, and the degree to which mass incarceration disproportionately affects communities of color. By creating a more just system, the First Step Act also creates a healthier country.

Scratch a sickness, and, chances are, you will find an underlying injustice. Consider the poor health experienced by victims of climate change. It may seem strange to think that injustice has anything to do with the storms, high temperatures, and other climate-related phenomena that cause so much injury, disease, and death around the world. Yet these challenges are much likelier to befall the vulnerable, socioeconomically disadvantaged populations that tend to live in low-lying coastal regions or in Caribbean small island developing states like Puerto Rico. It is a clear injustice that the costs of climate change are incurred by the haves, but borne by the have-nots.

Segregation also reflects injustice on a comparatively smaller scale, in cities. In a recent column, I wrote about how place shapes health, and how communities of color are likelier to live in crowded urban neighborhoods near major roadways. This can increase asthma risk in these communities, while cutting residents off from other parts of a city.

To say this is solely a flaw of urban design is to miss the injustice that created these unhealthy conditions. Residential segregation echoes the effects of the 20^th century practice of “redlining,” in which people of color were systematically denied loans in order to keep them out of certain neighborhoods. The legacy of this injustice can still be felt today, in the poor health of many urban communities.

Climate change and residential segregation show why we cannot talk about heath without talking about social and economic justice. Yes, it is the combined threat of storms, geographic disadvantage, and pollution that create much of the risk vulnerable communities face. But these hazards originate with the injustices of racism, economic inequality, and the unequal distribution of the material and political resources that can, when properly deployed, alleviate these challenges.

Why should we care about promoting justice? After all, a hallmark of injustice is that it does not affect everyone equally—at least not at first. Its initial victims are almost always the marginalized, the vulnerable, the people we too-often dismiss. Why should we worry about their plight if we do not share it? If we, personally, can avoid living in the rough parts of a city, or on a storm-vulnerable coast, or without enough money to insulate ourselves from a frequently dangerous and unfair world, what is our incentive for creating a more just society?

Over 50 years ago, Martin Luther King Jr. addressed this question while sitting in a jail cell in Birmingham, Alabama, where he had been confined after his arrest for protesting racial segregation in the city. There, he wrote a letter contradicting the idea that we can somehow compartmentalize injustice in our society by placing it out of sight and out of mind. “Injustice anywhere is a threat to justice everywhere,” he wrote. “We are caught in an inescapable network of mutuality, tied in a single garment of destiny. Whatever affects one directly, affects all indirectly.”

King was right. When we allow injustice to flourish, it is only a matter of time before it affects us all. This is especially true when it comes to health. It is true, for example, that the effects of climate change are currently felt most acutely by socioeconomically vulnerable communities, but this will not always be the case. If we ignore the injustice that puts their health at risk, we also jeopardize our own health, by allowing climate change to worsen, until the day comes when its effects will be felt equally by everyone.

The same is true of mass incarceration. As long as we pursue punishment at the expense of rehabilitation, we will continue to undermine the health of other people’s communities, until the day comes when we, inevitably, undermine the health of our own.

The fact that, in our polarized era, the First Step Act received bipartisan support is a testament to this reality. The problem of mass incarceration had grown so large that it became clear to all that it was in our collective interest to address it. But on so many issues we should not wait until the injustices that undermine health are as advanced as mass incarceration has become before we act.

Rather, we should heed what Dr. King called, “The fierce urgency of now,” and acknowledge that there can be no health without social and economic justice. As we approach Martin Luther King Jr. Day, it is a fitting tribute to his legacy, and a necessary condition for creating a healthier world, to address injustice whenever, and wherever, we see it.

Sandro Galea, MD, DrPH, is Professor and Dean at the Boston University School of Public Health. His latest book, Well: What we need to talk about when we talk about health, will be published in May 2019. Follow him on Twitter: @sandrogalea