Lung cancer kills millions of people worldwide each year. But the cancer cells that kill those people are usually their own.

For the first time ever, scientists think they might have found an exception in Colombia.

A 41-year-old Medellin man died this week from lung cancer, but researchers believe the cancer may have been passed to him by a type of tapeworm that commonly infects people in the developing world.

About 75 million people currently have the H. nana tapeworm, according to the Centers for Disease Control and Prevention (CDC).

“It looked like cancer, but the tumors were composed of cells that were not human,” says Dr. Atis Muehlenbachs, the lead researcher on the project with the CDC.

Doctors think that the man’s compromised immune system – he had HIV – allowed the cancer to take hold more easily.

The cells were much smaller than the typical human cancer cell, causing researchers to wonder if they were tapeworm cancer cells that had spread to the human host. After roughly two years of testing – the man first fell ill in 2013 – DNA from the cells confirmed that they were in fact tapeworm cells.

Tragically, the man died just days later.

The discovery has prompted questions as to whether or not non-human cancers may actually cause illness on a regular basis. Because the symptoms are similar, it’s possible that cases have been misdiagnosed in the past.

“We think this type of event is rare. However, this tapeworm is found worldwide”

“We think this type of event is rare,” said Muehlenbachs. “However, this tapeworm is found worldwide and millions of people globally suffer from conditions like HIV that weaken their immune system. So there may be more cases that are unrecognized.”

Because the H. nana tapeworm can complete its entire lifecycle within a human host, they can reproduce in large numbers. That can be particularly dangerous for people with weakened immune systems.

The tapeworm is spread when people ingest feces from infected people, bugs or rodents. The CDC recommends washing hands with soap and warm water before handling food and cooking raw vegetables and fruits thoroughly.