Some United States veterans who served in Vietnam in the late 1960s may have been infected by a slow-killing parasite that only shows its symptoms decades after its ingestion, a new study revealed.

Based on the research recently commissioned by the U.S. Department of Veterans Affairs in South Korea, scientists believe that there is a significant link between liver flukes and a rare bile duct cancer also known as cholangiocarcinoma, which has mysteriously affected hundreds of veterans.

Sung-Tae Hong, the tropical medicine specialist who oversaw the small pilot study in Seoul National University, stated that of the 50 blood samples of veterans with bile duct cancer they studied, 20% turned out positive or near positive for liver fluke antibodies.

Hong said that the preliminary results were “surprising” while admitting that they may include false positives as the research is still ongoing, according to the Associated Press.

The report was confirmed by Northport VA Medical Center spokesman Christopher Goodman, who revealed that the agency’s New York facility collected the samples and sent them to the lab. While he did not react to the preliminary results, he noted that those who tested positive were notified of the results.

While generally rarely found in Americans, the parasites have infected an estimated 25 million people worldwide. If detected early, the parasites can easily be cleared with pills but can live for decades without apparent symptoms if left untreated. The bile duct will eventually swell in time, which can lead to cancer. It is only during the final stages when symptoms start to emerge, with the patient losing weight, developing jaundice (yellow skin), itchy skin, and more.

It is believed that the soldiers ingested the parasite inside raw or undercooked fish they consumed while stationed in the Southeast Asian jungles almost half a century ago. By the time symptoms emerged decades later, patients are left with just a few months remaining to live while in debilitating pain.

One of those who tested positive was 69-year-old Gerry Wiggins, who was on active duty in Vietnam from 1968 to 1969.

“I was in a state of shock,” Wiggins said. “I didn’t think it would be me.”

He added that he participated in the research to help save lives as he had already lost some of his friends to the disease. After further tests showed he had two cysts on his bile duct, he had them removed before they could even develop into cancer.

“We are taking this seriously,” Department of Veterans Affairs representative Curt Cashour was quoted as saying. “But until further research, a recommendation cannot be made either way.”

A report from the AP last year reportedly generated attention to liver flukes and cancer-stricken veterans, sparking calls for broader research into the issue. According to the story, the VA has seen an estimated 700 veterans with cholangiocarcinoma in the past 15 years. Since most were not aware of its link to Vietnam, claims for service-related benefits were less than half of the infected patients. The AP also noted that 80% of the claims were rejected.