As erectile dysfunction drugs extend the sex lives of people older than 50, doctors are failing to diagnose HIV among sexually active seniors, according to a study published by the World Health Organization (WHO) and reported on by Reuters.

“Screening is less common for older adults, who are assumed not to be at risk,” wrote study authors from the WHO and Minnesota’s St. Olaf College. “HIV prevalence and incidence in the over-50-year-olds seem surprisingly high, and the risk factors are totally unexplored.”

According to the article, patients older than 50 make up about 11 percent of new HIV diagnoses in the United States and 8 percent in Europe. Researchers say it is crucial to diagnose and treat HIV early among older adults—particularly those older than 65—because their immune systems are already weakened with age; this shortens their life expectancy to only four years after contracting the virus if not treated.