Late on Easter Sunday afternoon, we heard four quick pops. One neighbor said it was fireworks, but a minute later people gathered around a boy lying on the sidewalk. He was unresponsive, the wire of his retainer visible behind his motionless lips. I tried to find a pulse, but there was none. Two minutes later, this beautiful 16-year-old boy was in the back of a patrol car rushing to the ER.

Two days later, we learned that William Bethel had died.

Our hearts go out to his family, friends, teachers, and everyone who knew this highly regarded young man. I can barely imagine their pain.

The questions that haunt me — and should haunt all of us — are: Why did this happen, and what can we do to prevent the loss of another promising child of Philadelphia? As a health expert, my job is to study these issues from a safe distance. On that Sunday, the impact of my work got very real. Because violence is a significant health issue, maybe my role here is to try to explain the unexplainable and understand why tragedies like these keep happening.

I can't presume to know what happened on that Easter day on South Street, just hours after families (and the mayor) paraded down that very same street. All we know is what's been reported; that two groups of kids argued and one fired a gun, killing William.

We can't know what insult made the shooter so angry (I assume) that he had to pull out his gun, or why he felt it necessary to bring it with him in the first place, or why he lacked the self-control not to fire, or what had happened in his life to put all that explosive rage so close to the surface.

We do know that violence does not happen in isolation; it's the result of circumstances and choices made at the individual, community, and government levels.

Research tells us that the events of our earliest years live with us until the day we die. Serious early childhood experiences like abuse, a parent's death, or divorce can cause illness and early death. They can also prime us for risk-taking and impulsive behavior.

Health-care providers have started to ask "What happened to you?" before "Why do you do this?" to assess and prevent the impact of these adverse experiences.

Where we live can impact our health more than the traits we inherit from our parents. Essentially, zip codes matter more than genetic codes. Like individuals, whole communities can be traumatized. Harmful experiences now and from generations past can weigh on the collective psyche, holding back people's progress and priming them for stress-related illness.

Institutional racism, chronic unemployment, poverty, limited opportunity, disinvestment, and loss of economic resources destroy the infrastructure and sap a community's strength, denying its children the opportunities other neighborhoods provide.

Again, I don't know why the shooter did what he did, but I do know that no child should carry a loaded weapon. I know that lax gun laws allow these guns to circulate. I know that our culture glorifies gun violence and that in the face of the NRA-led gun lobby, our political leaders are impotent to act. And I know that the mythical "good guy with a gun" would never have stopped this.

Philadelphia is the poorest of the nation's biggest cities. While some of our neighborhoods are growing and young people are moving in, others are still struggling from decades of stagnation and chronic social issues.

But I believe there is much we can do. Something as simple as fixing up a vacant lot can decrease gun violence. Clean streets, jobs, well-kept homes, and good schools signal to people — especially young people — that somebody cares. Pride is contagious.

Unfortunately, violence is also contagious, infecting younger men in greater numbers. Boys need to find their place in the world, a meaningful place where adults take an interest in them and they feel valued. They need to learn how to be real men, strong in values and passionate in their conviction to do right, not some form of soulless, toxic masculinity. They need constructive challenges to surmount to prove their worth and develop resilience.

Our job as adults — especially adult men — is to be engaged mentors and help them grow with love and hope. Boys' Latin, the school William Bethel attended, seems like just such a place. If only every child had the same opportunity.

The children of Stoneman Douglas High School in Parkland, Fla., turned their anger and rage about a senseless school shooting into social action and a nationwide antiviolence movement. Instead of fighting boredom, frustration, or each other, they are fighting for change. If only those boys on South Street that Sunday had something better to fight for.

Drew Harris is a professor of population health and health policy at Thomas Jefferson University.