Hundreds of Vietnam War veterans may have been infected by slow-killing parasites while fighting in Southeast Asia decades ago and not even know it, a new study suggests. Research commissioned by the Department of Veterans Affairs last spring found a link between a rare bile-duct cancer recorded among veterans and liver flukes—parasitic worms found in the rivers of Vietnam. According to Sung-Tae Hong, a tropical-medicine specialist who carried out tests as part of the study, 20 percent of 50 blood samples taken from veterans came back positive or near-positive for liver fluke antibodies, the Associated Press reports. Veterans who’ve been infected by the parasites often experience no symptoms, and may not be diagnosed with the rare form of cancer until they have only a few months to live. “I was in a state of shock,” said Gerry Wiggins, one of the veterans whose blood sample came back positive. “I didn’t think it would be me,” he said, adding that he’d already lost friends to the disease.