Fifteen years ago, at 8:46 a.m. on September 11, 2001, American Airlines Flight 11 crashed into the North Tower of the World Trade Center, followed, 17 minutes later, by United Airlines 175, crashing into the South Tower. Two other planes, American Airlines 77 and United Airlines 93, were also hijacked that day, crashing respectively into the Pentagon and into a field in Pennsylvania. Nearly 3,000 people died on September 11, 2001, and about 6,000 people were physically injured. The immediate aftermath of the attacks cost at least $10 billion in property damage and about $3 trillion in total costs. The long-term global consequences of the attacks continue to be felt to this day—9/11 resulted in the launch of global wars in Iraq and Afghanistan, with about 500,000 people dying in Iraq and likely a comparable number of deaths in Afghanistan. In many ways, 9/11 changed everything: We continue to face what feels like an interminable string of terrorist attacks, that kill dozens of people at a time, despite a wholesale upgrading of security measures worldwide.

In an 1818 letter to his brothers, a former medical student, and burgeoning poet, named John Keats described what he considered to be the most important quality possessed by “a man of achievement.” He called it “negative capability” and defined it as “when a man is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason.”

For millions of people, Sept. 11, 2001, was the first mass disaster experienced in real time. Many who watched the event unfold on television can still remember with great clarity, as the 15th anniversary approaches, where they were, and how they felt, on that terrible day.

I was in New York City on 9/11, just starting my career as an epidemiologist. Along with countless other New Yorkers, I watched with horror as the World Trade Center towers collapsed. Stunned by the destruction, our research team quickly became concerned with the potential long-term mental health consequences of the attacks.

The U.S. has substantially worse health outcomes than all its peer nations. In a list of non-communicable disease mortality in 17 high-resource countries, the U.S. has the second-highest death rate at 418 deaths per 100,000 people.

Next week marks the 11th anniversary of Hurricane Katrina, which swept through the Gulf Coast of the United States. The storm breached levees, led to extensive flooding, and displaced hundreds of thousands of people in Alabama, Louisiana, and Mississippi. The hurricane would ultimately kill more than 1,800 people, and inflict over $100 billion in damage. Particularly hard hit was the city of New Orleans.

The issue of gun violence has reached, it seems, epidemic proportions. Since taking office President Barack Obama has addressed the nation 14 times in the wake of a mass shooting. In June, House Democrats staged a sit-in to protest the lack of national legislative movement on gun control. Despite these efforts, there has been little progress on this issue at the federal level, and the partisan divide remains as wide as ever. Presidential candidate Hillary Clinton supports a range of gun reforms, including an assault weapons ban, while candidate Donald Trump calls gun bans “a total failure.”

In recent years, you may have heard the phrase “the first child to live to 150 has already been born.” It is an exciting thought. Regardless of whether or not it turns out to be true, it is a fact that global average life expectancy has risendramatically over the last century. According to the World Health Organization, it increased by five years between 2000 and 2015 alone. The United States is no exception to this trend. Last month, the Centers for Disease Control and Prevention reported that, between 2000 and 2014, overall life expectancy in the US increased by two years. This advance is in keeping with prior national life expectancy gains, steadily trending up. In 1900, US life expectancy was about 47 years. It is now close to 79 years. Given this increase, it seems reasonable to expect that our children will live longer lives than we will, lives characterized by significantly greater wellbeing. But is this really the case?