Health researchers have some grim news for Americans: We are dying younger, and life expectancy is now down for the second straight year — something not seen in more than half a century.

One undeniable culprit is the opioid epidemic, which is cutting down young adults at alarming and increasing rates, the researchers say.

The numbers are “disturbing,” said Robert Anderson, chief of the mortality statistics branch of the National Center for Health Statistics. The branch is part of the federal Centers for Disease Control and Prevention, which released two reports Thursday. One focused on all causes of death and the other zeroed in on drug overdose deaths.

A baby born in the United States in 2016 could expect to live 78.6 years, a decrease of more than a month from 2015 and more than two months from 2014. That’s the first two-year decline since 1962 and 1963 when spikes in flu deaths were likely to blame, Anderson said.

Before 2015, the last one-year decline was in 1993 and was attributed partly to the AIDS epidemic.

The declines are shockingly out of sync with a larger world in which lives are getting longer and healthier, public health experts said.

“The rest of the world is improving. The rest of the world is seeing large declines in mortality and large improvements in life expectancy,” said Peter Muennig, a professor of health policy and management at Columbia University. “That’s true in rich countries and middle-income countries and generally true even in lower-income countries.”

The difference between the U.S. and most of the rest of the world “is very stark,” said Jonathan Skinner, a professor of economics at Dartmouth College.

Newborns in 29 countries, including Japan, Australia and Spain, had life expectancies above 80 years in 2015, according to the World Health Organization. The average global life expectancy was 71.4 and rising, according to that agency’s most recent report.

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So what’s going wrong with American health? At first glance, the new statistics present a paradox: Overall death rates for the nation actually fell in 2016, and so did deaths from seven of the 10 biggest killers, including cancer and heart disease. But life expectancy fell, too — because death rates ticked up in people under 65.

“Every time you lose a young person, you lose many more years of life than when you lose an old person,” Skinner says.

In fact, the report contains reassuring news for older Americans: If you make it to age 65, you can expect to live another 18 years if you are a man and 20.6 years if you are a woman. While most deaths still occur in older people, older people are dying at a slower rate.

But fewer people are making it to 65. And the biggest killers of young people include what statisticians call “unintentional injuries” — a category that covers drug overdoses, traffic crashes and falls. Deaths from those causes rose 9.7% in 2016.

A second CDC report makes it clear that drug overdoses are driving that wave of premature deaths, killing 63,600 people in 2016. The death rate from overdoses tripled from 6.1 per 100,000 people in 1999 to 19.8 in 2016.

And it spiked 21% from 2015 to 2016, the report says.

The fatal drugs increasingly include synthetic opioids such as fentanyl, along with heroin and other opioids, the report says.

The opioid epidemic is not the only explanation for falling life expectancy, Anderson said. Stalled progress on the nation’s biggest killer, heart disease, is playing a long-term role, despite a decline in deaths in 2016, he said.

Everything from bad roads to bad diets to unequal use of health care contributes to the death gap between the United States and other rich countries, Muennig said.

Suicides also increased in 2016, as did reported deaths from Alzheimer’s disease.

The overall decline in U.S. life expectancy cannot yet be called a trend, Anderson said: “I hope it’s just a two-year thing.” But, he said, the picture is unlikely to improve if the rise in drug deaths is not stopped.

“So my guess is that is that when all is said and done, we are probably going to see something similar for 2017.”