Hundreds of veterans from the Vietnam War may be slowly dying from a parasite picked up during their time in the Southeast Asian country.

A new study shows that more than 20 percent of Vietnam veterans’ blood samples — out of a total of 50 — came back with antibodies for liver flukes, which can cause tremendous pain and may lead to bile duct cancer, but only decades after infection, the Associated Press reports.

Based on the way this specific kind of parasite works, veterans may not realize they’re infected until the symptoms start to appear. By then, however, it’s almost too late, as the infection is already at an advanced stage.

Sung-Tae Hong, who carried out the study at Seoul National University in South Korea on behalf of the Department of Veterans Affairs, said he was shocked at the results.