Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Tapeworms are bad enough. They get inside people, lay their eggs, and cause symptoms such as diarrhea and weakness. And they can infest a body for a lifetime.

But doctors were stunned to find out they can do something worse: infect people with tumors.

“We were amazed when we found this new type of disease — tapeworms growing inside a person essentially getting cancer that spreads to the person, causing tumors,” said Dr. Atis Muehlenbachs, a pathologist at the Centers for Disease Control and Prevention who helped figure out what happened.

“In January 2013, a 41-year-old man in Medellín, Colombia, presented with fatigue, fever, cough, and weight loss of several months’ duration. He had received a diagnosis of HIV infection in 2006,” Muehlenbachs and colleagues reported in the New England Journal of Medicine.

“We were amazed when we found this new type of disease — tapeworms growing inside a person essentially getting cancer that spreads to the person, causing tumors."

The man didn’t take his HIV medication and he was very heavily infected with dwarf tapeworms, known scientifically as Hymenolepis nana.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

The parasites, which can grow up to an inch and a half long, don’t usually cause symptoms. But they can reproduce inside people for years. As many as 75 million people globally carry them.

“People get the tapeworm by eating food contaminated with mouse droppings or insects or by ingesting feces from someone else who is infected,” CDC said.

The patient was very ill and his stool was full of tapeworm eggs, the international team of researchers reported. And his lungs and lymph nodes were full of tiny little tumors like no one had seen before.

“The tumors looked similar to a human cancer, but initial CDC lab studies revealed the cancer-like cells were not human,” the CDC said in a statement.

Maybe it was a slime mold, the team thought. As they investigated, the patient got worse and he died a few months later. A barrage of tests showed the little clusters of tumor cells had the same DNA as the tapeworms.

“The tumors looked similar to a human cancer, but initial CDC lab studies revealed the cancer-like cells were not human."

It’s the first time a parasite has been found to spread cancer. Viruses and bacteria can cause cancer — the human papillomavirus (HPV) causes cervical and anal cancer and Helicobacter pylori bacteria cause stomach cancer. But no one thought a tapeworm could do it, until now.

“We think this type of event is rare,” Muehlenbachs said.

“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. It’s definitely an area that deserves more study.”

It could even be that tapeworm infections in people get mistaken for cancer, the researchers said. Tapeworms can be eliminated with drugs, but it’s not clear if those drugs would clear up tumors. It’s also not clear if chemotherapy drugs would work against them.

No one ever knew tapeworms could get cancer, either, but the cells were clearly malignant.

“The host–parasite interaction that we report should stimulate deeper exploration of the relationships between infection and cancer,” the researchers concluded.