Tapeworms 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.
As many as 75 million people globally carry the parasite.