A woman crouches on the sidewalk next to her boyfriend, who is unresponsive and not breathing after an opioid overdose in the Boston suburb of Everett, Massachusetts, on August 23, 2017.

In 2016, 64,000 people died from drug overdoses in the U.S., most of them due to opioids. That’s more than the number of Americans killed in the wars in Vietnam and Iraq combined.

Three factors led to those numbers, Nora Volkow, the director of the National Institute on Drug Abuse, a part of the National Institutes of Health, said at the Spotlight Health Festival, which is co-hosted by The Aspen Institute and The Atlantic. First, the epidemic was started by a healthcare system that sought to minimize pain and suffering. Physicians were taught that those with pain wouldn’t get addicted to pain medication, she said. “Unfortunately, those beliefs were completely wrong,” she said. What it resulted in was “an overprescription of opioids,” Volkow said.

Second, Volkow said, this overprescription coincided with a massive flood of heroin in the 2000s from Mexico. Deaths from heroin overdoses had been consistent for years—about 2,000 people died of overdoses from the drug each year, but in 2016 that number spiked to 15,000.

Thirdly, she said, heroin began to be laced by drug dealers with synthetic opioids, which are not only relatively easy to produce, but also stronger than heroin. Fentanyl, for example, is 50 times more potent than heroin. Carfentanil, another synthetic opioid, is 500 times more potent than heroin. Both synthetic opioids are pain relievers, but are also sold illegally as drugs. They are so powerful, Volkow said, that they get into the brain quickly. First responders are often unable to save those who have overdosed.