People aged 50 and older comprise a growing percentage of HIV patients in Europe, accounting for one in six new cases in 2015, researchers said Wednesday.

The older cohort was more likely to be diagnosed with advanced stages of HIV and AIDS compared to 15-49 year olds, and to contract the disease through heterosexual sex, they reported in a study, published in The Lancet.

Historically, the two groups most at risk have been gay men and intravenous drug users who share needles.

"Our findings suggest a new direction in which the HIV epidemic is evolving," said lead author Lara Tavoschi.

"They show the need to ensure all ages are appropriately targeted by sexual health services."

In 2013, UNAIDS estimated that 4.2 million people aged 50 and older were living with HIV worldwide, a fifth of them in Europe or the United States.

Higher infection rates among this slice of the population, especially in wealthy nations, is due to greater life expectancy among HIV carriers and as well as a jump in new cases.

Tavoschi, a researcher at the European Centre for Disease Prevention and Control in Sweden, looked at European health data from 2004 to 2015.

During that period, the number of older people diagnosed with HIV increased in 16 European countries, while remaining stable or declining in 15 others, the study found.

Overall, the rate of new cases among people aged 50 or older across 31 countries -- the European Union along with Iceland, Liechtenstein and Norway -- went up, on average, by more than two percent each year.

It rose from 2.1 to 2.6 people per 100,000.

The rate among older men was four times higher than for older women.

- Lifelong treatment -New cases among young people, meanwhile, remained steady, averaging about 11.4 additional patients per 100,000 people each year from 2004 to 2015.

"Our findings illustrate a clear need to provide comprehensive HIV prevention programmes... targeted towards older adults across Europe," Tavoschi said.

Among the measures needed are access to condoms, better testing opportunities -- including self-testing kits -- and fast-track treatment, she said in a statement.

"This should help to prevent further transmission and lower the risk of severe health complications, which is of utmost importance among older adults living with HIV as their risk of mortality is higher."

In countries where the number of new HIV cases among older people rose from 2004 to 2015, the average annual rate of increase was highest in Lithuania (14.4 percent), Latvia (12.1 percent), the Czech Republic (12 percent), and Estonia (10.1 percent).

In other European countries the rate was Belgium (3.9 percent), Germany (8.1 percent), Ireland (5.4 percent) and Britain (3.6 percent).

By 2015, rates of new HIV infection were highest in Estonia (7.5 cases per 100,000 people), Latvia (7.17), Malta (7.15) and Portugal (6.0).

The country with the lowest incidence of new HIV cases among those 50 and older was Slovakia.

Since the AIDS epidemic erupted in the 1980s, 76.1 million people have been infected with HIV, the virus that causes AIDS. Some 35 million have died.

Last year, AIDS killed a million people and infected another 1.8 million, according to the United Nations.

Infections and deaths are on the decline, but the number of people living with HIV -- requiring lifelong treatment -- continues to grow.