LA BLANCA – A rare parasite is causing complications for veterans who fought in the jungles of Vietnam.

Forty years since the Vietnam War and riding his bike, veteran Richard Pena would rather look forward.

"I really don’t like to talk about it," says Pena. "It’s not, how can I tell you this, it’s not a good experience."

Troubling news, linked to the past, is bringing back difficult memories for Pena. The America’s Last Patrol Commander is learning of a silent killer.

A microscopic parasite found in fish is infecting some Vietnam veterans.  It’s slowly picking off Pena's fellow comrades.

"It's a shame," said Pena.

The Department of Veterans Affairs is conducting a pilot study looking at how the parasitic liver fluke found in Vietnam is possibly leading to a deadly cancer in Vietnam veterans.

The parasite is found in raw or undercooked fish, something Pena said the troops were warned not to eat in Vietnam.

"You would get advised, don’t eat the food. You’ve got C-rations, but never trust enemy’s food," said the veteran.

The Centers for Disease Control said the parasite moves from the fish to a human host. It can cause damage for years. If left untreated, the CDC says it leads to cancer.

The Press Secretary for the Department of Veterans Affairs, Curt Cashour, issued this statement to CHANNEL 5 NEWS.

"Blood samples taken from 50 veterans were sent to a lab in South Korea, the only lab in the world capable of testing for liver fluke. Some samples have tested positive and we have notified all of those Veterans.”

Cashour went on to say they’ve received 240 claims for bile duct cancer since 2013.

He says a quarter of the claims for bile duct conditions were granted. It’s not clear if they are all linked to service in Vietnam.

For Pena, it’s not the first time hazards of war are causing horrific side effects. He said he lost a brother in arms to Agent Orange.

"It kills you, slowly, but surely you get there. The effects will come around," says Pena.