The latest official U.S. life expectancy numbers paint a relatively grim picture for the nation. For the first time in a decade our death rate increased from the year before; 2015 saw roughly 86,000 more deaths than 2014, according to the new report. The National Center for Health Statistics (NCHS), which released the numbers this week, found that in 2015 the death rate jumped 1.2 percent from 724.6 deaths per 100,000 people in 2014 to 733.1. The agency calculated that this spike pushed life expectancy down, too. Standard life expectancy at birth dropped to 78.8 years from 78.9 just a year earlier. Preliminary analysis suggests the increase in deaths may have been driven by drug overdoses and an unusually severe flu season in early 2015, which may have exacerbated potentially fatal conditions such as heart disease.

For insights into the reasons for the startling new figures Scientific American spoke with Robert Anderson, chief of the Mortality Statistics Branch at the NCHS.

[An edited transcript of the interview follows.]

You already have some preliminary numbers from state death certificates about the first half of 2016. Do they suggest that the increase in deaths in 2015 was just a blip?

Maybe. It’s really difficult to say, but our indications based on the provisional data suggest it may be a blip. We only have numbers for about the first half of 2016, so we do have to be careful about interpreting them. But it appears that if we look at the age-adjusted rate for 2015, which is about 733 deaths per 100,000 population, and look at the age-adjusted rate for the 12 months ending in June 2016, it drops back to 720 per 100,000—and that’s actually below the 2014 level, so it may very well be a blip. With that said, we still have the quarter three and quarter four of 2016 to come. We just entered the flu season now and things could be bad from a flu standpoint, which could drive the numbers up.

There were roughly 86,000 more deaths in 2015 than in 2014, with a variety of causes listed. Is there any reason there would be more deaths in 2015 from, say, Alzheimer’s and diabetes than in years past? What’s behind these increases?

I wish I could explain them. There are certain aspects of mortality that are predictable and preventable, and certain aspects that are essentially random and unpredictable. With a one-year increase we really don’t know if we are looking at something that is predictable and preventable or whether it’s random.

The flu could be an explanation here and could explain across multiple causes of death, but it’s hard to say for sure. The flu can impact other causes of death, and it can cause people with existing chronic conditions to die from those conditions. So someone with heart disease who gets the flu, that flu can precipitate a heart attack or exacerbate existing chronic lung disease or many other things. For people who are very ill and may be hanging on, they can die sooner than they may have otherwise.

Is there any indication that flu was a big factor for 2015?

We won’t know for sure until we do some more thorough analysis. For now we can look at flu mortality surveillance, which is an indication if something is going on. If you look at the early part of 2015 in the middle of the flu season, there is quite a large spike. If you look at the tail end of 2015 in the next flu season, it is close to the baseline, so it seems like relatively mild flu season at the end of 2015—but pretty severe early that year—so that could partly explain what we’re seeing. But to be more definitive about it we would need the flu experts to weigh in.

In 2015 there were 240 more infant deaths than in 2014, and it is striking that there was an 11.3 percent increase in infant deaths due to “unintentional injuries.” I know that that official classification does not include sudden infant death syndrome, which also had a slight increase. So what are “unintentional injuries,” and how can we prevent them?

It appears the increase in unintentional injuries here is accidental suffocation—both in bed and not specified as such. We will be getting more into that soon.

So for adults, beyond flu, what were key drivers that we know about?

We know that for adults the big culprit from 2014 to 2015 is increases in drug overdose. That’s another thing we’ll be diving into in the coming weeks and we’ll have a report on that, probably in January, to detail that information. We have data on accidental poisonings overall—and the overwhelming majority of those are drug related—so that will give you an idea about what is going on with this. The accidental poisonings, which include drugs, alcohol poisoning and poisoning due to other toxic substances—that total number for 2015 is 47,478, and for 2014 it was 42,032. It’s really a big difference. And the overwhelming majority of those are drug-related overdoses.

When was the last U.S. death rate increase?

Last time we saw an increase was from 2004 to 2005, and it just bumped up a tiny bit. We figured there that it was related to flu. This is a little bit bigger increase than from ‘04 to ‘05.

Right, so how can we explain 2015?

The drug overdoses are a big part of this. But we also saw increases in heart disease and stroke mortality. The heart disease probably affects this more than anything else. For heart disease, the increase between the two years—2014 to 2015—was from 614,000 roughly to 633,000, so that’s almost 20,000 deaths due to heart disease. Part of that is due to the aging of the population, but the heart disease rate can be affected by the flu. Some people are also saying obesity is a possible explanation for those heart disease numbers—and maybe it is. I don’t know that there is any solid evidence that that’s what is going on here though.

Are we dying younger from heart diseases?

I don’t think so. We would have to take a closer look to see what the average age of death is for heart disease and whether it’s changed or not. I don’t think we have seen any dramatic shifts or any one-year shifts. The average age at death has obviously been increasing over time. I think we’ve gotten pretty good at keeping people with heart disease alive.

Is there any good news in these 2015 numbers?

Cancer mortality continued to decline, which is good. But that’s about the only thing I can think of.