Cancer is on the rise among younger people and scientists don’t really know why, according to research published in BMJ Oncology on Tuesday, an unexplained trend that threatens to strain already stretched healthcare systems and upends much of what we thought we knew about the disease.

While cancer still tends to be more common among older people, the number of cancers among people younger than 50—when it is considered to be “early-onset cancer”—has risen over the past three decades, according to the peer reviewed analysis of health data.