Since 1990, the number of Americans who have died every year from car accidents...

Show me how I did. I don't want to play; just tell me the answers. … has dipped, risen slowly but then declined, in part as a result of safer cars and improved road safety. Recessions and increasing gas prices can sometimes lead to a temporary drop in car accident fatalities, as seen in the late 2000s. Deaths are on the rise again recently, perhaps because of more distracted driving.

Since 1990, the number of Americans who have died every year from guns...

Show me how I did. … has been slowly rising after a drop in the 1990s. Gun deaths, most of which are suicides, increased by 8 percent in 2015, the largest single-year jump since the C.D.C. began keeping computerized death records.

Since 1990, the number of Americans who have died every year from H.I.V....

Show me how I did. … has spiked and then plummeted. By the mid-1990s, over 40,000 people were dying from H.I.V. each year, but death rates fell sharply with the development of antiretroviral treatments. Nevertheless, H.I.V. remains a leading cause of death for those 25 to 44, particularly among black men.

Since 1990, the number of Americans who have died every year from drug overdoses...

Show me how I did. … has increased by more than 650 percent. In 2015, more Americans died from drug overdoses than from car accidents and gun homicides combined.

It’s the worst drug overdose epidemic in American history, spurred by rising drug abuse, increased availability of prescription opioids and an influx of potent synthetics like fentanyl and carfentanil. Drug overdoses are now the leading cause of death for Americans under 50.

“It’s horrifying,” said Dr. Dan Ciccarone, a heroin researcher and a professor of family and community medicine at the University of California, San Francisco. “It’s not even the magnitude — it’s the steepness at which it’s climbing.” Preliminary numbers for 2017 suggest that overdose deaths are growing at a rate comparable to the height of the H.I.V. epidemic.

The number who die each year from...

The opioid epidemic has not fallen equally on all races or regions. Like an infectious disease, drug overdoses have emerged in clusters around the country.

Among those 15 to 44 — the age group in which drug overdose accounts for the greatest share of deaths — there are vast differences across racial categories and between urban and rural places. Despite the perception of the epidemic as primarily afflicting the rural working class, drug overdoses account for a greater percentage of deaths among the young in large cities and their suburbs, with urban and suburban whites most at risk.

Percentage of deaths in 15-to-44-year-olds caused by drug overdoses Note: In counties with fewer than 10 drug overdose deaths, the map combines observed totals with modeled estimates

The crisis has hit particularly hard in New England and in parts of Ohio, New Jersey, Pennsylvania and West Virginia. Those are the places where fentanyl, a potent synthetic opioid, started to flood into the heroin supply five to 10 years ago. Bodies began piling up.

“It just blew through all the measures we’d been taking,” said Dr. Alex Walley, the director of the Boston University Addiction Medicine Fellowship program. Fentanyl, which acts on opioid receptors in the brain but isn’t made from the opium poppy, is up to 50 times more powerful than heroin. Because of that potency, Dr. Walley said, there’s less time after an overdose for someone to intervene. The drug’s short half-life means that users have to inject more frequently, increasing the risk of transmitting H.I.V., hepatitis and other blood-borne diseases.

The illicit fentanyls being added to the heroin supply also vary widely in purity and quality. This unpredictability kills: Users often don’t realize the strength of what they’re taking until it’s too late.

An evolving epidemic

The country's struggles with opioids go back centuries, but the current epidemic has roots in the 1980s, when a few studies made doctors less wary of prescribing opioids for pain. Pharmaceutical manufacturers began aggressively marketing them for many ailments, particularly after OxyContin, a controlled-release opioid, hit the market in 1996. Soon, narcotics that had been reserved for cancer patients and end-of-life care found their way into medicine cabinets across the country.

Michael Botticelli, the executive director of the Grayken Center for Addiction Medicine at Boston Medical Center and former White House drug czar, said that rampant over-prescription of painkillers had actually been decreasing in the last few years. “We’d turned a corner on that,” Mr. Botticelli said, “but we’ve seen the continued escalation of overdose deaths in many states.”

Some experts argue that the crackdown on opioid prescriptions, undertaken when addiction treatment was insufficient, pushed people toward heroin and inadvertently drove up the rate of overdose deaths. Mr. Botticelli said that theory isn’t supported by the data.

Regardless of the cause, there is little disagreement that heroin addiction has evolved into a full-fledged crisis of its own. “Young people are flocking to heroin,” Dr. Ciccarone said. Halting prescription opioid abuse will probably not be enough to stop the growing number of deaths.

What can be done

The Trump administration has established a commission, led by Gov. Chris Christie of New Jersey, to study responses to the epidemic. The commission’s final report is expected in November.

Among public health researchers, there is broad agreement about what's needed: access to addiction treatment and medications; tighter regulation of prescription opioids; widespread distribution of naloxone, a fast-acting overdose antidote; needle exchanges or supervised injection centers; and law enforcement efforts to reduce the supply of heroin and illicitly manufactured fentanyl, the bulk of which is thought to come from China and Mexico.

“This is not a hopeless situation,” said Dr. Tom Frieden, the former director of the C.D.C., describing interdiction as a critical piece of the strategy. “If law enforcement can substantially interfere with the supply of illicit opioids, it will save a lot of lives.”

He and other experts expressed hope for new partnerships between law enforcement and public health. Cooperation would, for example, allow public health workers to pinpoint groups of drug users who may be amenable to treatment or identify particularly deadly batches of drugs before they kill.