Centers for Disease Control researchers say it is one of the strangest and most unsettling cases they have ever encountered. The researchers say that scans of a very ill man in Colombia revealed what "looked like cancer, but the tumors were composed of cells that were not human," NPR reports. The tumors had come from the man's resident tapeworm, which had developed cancer and somehow spread the disease to its host, according to the scientists, whose study published in the New England Journal of Medicine describes the "invasion of human tissue by abnormal, proliferating, genetically altered tapeworm cells" as a "novel disease mechanism." The man died 72 hours after researchers pinpointed the cause of the tumors, Live Science reports.
The Colombian man had HIV, which meant the tapeworm's growth in his body was not halted the way it would have been by a healthy immune system, Live Science notes. A CDC pathologist tells the Washington Post that it took dozens of tests to reveal the cause of the illness, and finding tapeworm DNA in the tumors was a huge surprise. "This is the first time we've seen parasite-derived cancer cells spreading within an individual," he says. "This is a very unusual, very unique illness." The Post notes that the study raises questions about what other parasites dwelling in the human body can develop—and spread—cancer. (For more tapeworm horror, read about what caused this man's headache.)