Story highlights Death rates haven't gone up since 2005, which was a bad flu season

More people are dying from Alzheimer's, suicide, homicide, firearm deaths, hypertension and drug overdoses

It's too soon to know whether it is a trend

(CNN) Americans died at a higher rate in 2015, news that comes as a surprise to experts who are used to seeing a decline in the number of deaths.

The age-adjusted death rate was 729.5 deaths per 100,000 people in 2015, compared with 723.2 in 2014.

That might not sound like much, but an increase in the overall rate is incredibly rare. The last time we saw significant upticks in the death rate was 2005, when there was an especially bad flu season. There was also an increase in 1993, but that was in the pre-protease inhibitor era, when more people were dying from HIV. There was a small increase in 1999 as well.

It may be too early to tell what's driving the current trend, which is based on preliminary research gathered by the National Center for Health Statistics from last year's death records. Death rates have improved over the years due to advances in modern medicine, public safety improvements such as seat belt laws and a decline in the number of people who take up smoking. Better disease management has also helped.

Though cancer deaths have stayed about the same, last year's uptick in deaths seems to be concentrated in a handful of diseases. There were more deaths in people who have Alzheimer's, chronic lower respiratory disease, Parkinson's, stroke, septicemia and hypertension. Deaths from preventable causes, such as firearm deaths and homicides, seem to be up. There were also more suicides, drug overdoses, falls and accidents.

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