Out of 35 million people living with HIV around the world, an estimated 4.2 million are older than 50, aidsmap reports. Publishing their findings in the journal AIDS, researchers at UNAIDS used the agency’s prevalence figures to estimate prevalence among older adults. Then they compared the figures with household surveys and an examination of long-term trends in HIV infection data in order to assess their accuracy.

Out of the estimated 4.2 million people older than 50 who were living with HIV in 2013, 2.5 million were in sub-Saharan Africa.

HIV prevalence has steadily risen in all five of the five-year age brackets over 50 since 1995, more than doubling among people ages 50 to 54, increasing by a third among those ages 55 to 59, and rising by a quarter among those 60 to 64.

The household surveys generally fell in line with the UNAIDS estimates.

Two possible reasons for the rise in prevalence among older adults: People are living longer with the virus thanks to antiretroviral treatment, and there may be a high rate of new infections among people later in life.

To read the aidsmap story, click here.

To read the study abstract, click here.