Appendix cancer is a condition that, until recently, was so rare that most people never gave it a second thought. For decades, it was the kind of disease that doctors might encounter only once or twice in a career, and it was almost always found in older adults, reports The Conversation.

The appendix is a small, finger-shaped pouch attached to the large intestine. Its purpose in the body is still debated, but it is best known for causing appendicitis, a painful inflammation that often requires emergency surgery. What’s less well known is that cancer can develop in the appendix, usually without any warning signs.

A new study, published in Annals of Internal Medicine, has shown that the number of appendix cancer cases has increased dramatically among people born after the 1970s. In fact, the incidence has tripled or even quadrupled in younger generations compared with those born in the 1940s.

While the overall numbers are still small—as appendix cancer affects just a handful of people per million each year—the rapid rise is striking. Even more notable is that about one in three cases now occurs in adults under 50, a much higher proportion than seen in other types of gastrointestinal cancers.