Let us never assume that we know anything about anything—especially cancer. Medical understanding of the disease was challenged again this week by a case study published in the New England Journal of Medicine that describes the story of an HIV-positive man who died of cancer he contracted from a tapeworm.

The tapeworm in question, Hymenolepis nana, was uniquely qualified for the job. H. nana is the world’s most common human tapeworm, infecting as many as 75 million people. The parasite’s presence usually doesn’t produce any symptoms; most people don’t even realize they’ve got one.

In addition to its stealth, H. nana has a special talent: sticking around. Every other tapeworm hatches in one host, and then needs to move into another to grow and reproduce. Not H. nana. It can stay put for generations, especially in people with fragile immune systems.

A fragile immune system was what brought the 41-year-old Colombian man to the doctor in the first place. The man had been diagnosed with HIV seven years earlier, but for one reason or another, he had not kept up with his medication. As a result, his immune system was weakened and he was especially susceptible to infection. By the time he came to the clinic, the man was feverish, fatigued, and coughing, and had steadily been losing weight for months.

Doctors found H. nana eggs in the man’s stool, confirming an infection. Tests of his lungs, liver, lymph nodes, and adrenal glands revealed a constellation of tumors ranging in size from 0.16 to 1.7 inches, and biopsies of the tumors revealed something very strange. The tumor cells were malignant and growing, but they were way, way too small to be human. DNA tests confirmed it: The man definitely had cancer … but it wasn’t his. It was his tapeworm’s. The patient passed away just three days later.

To say that this case is unusual would be an understatement. Contagious cancers do exist in dogs and Tasmanian devils, and this year scientists discovered a form of contagious clam leukemia. But as far as we know, cancer doesn’t pass from one species into another. Humans don’t get other animals’ cancer. 

This case was special, says Tommy Leung, a parasitologist at the University of New England. “I don’t think anyone has seen anything like it before,” he said in an email to mental_floss. Leung believes the cancer transmission was possible due to a confluence of three factors: the tapeworm’s endurance; the patient’s compromised immune system; and a mutation in the H. nana larva that transformed run-of-the-mill cancer into an infectious disease.

So should we start freaking out about tapeworm cancer? Absolutely not. It’s possible that this has happened before and been overlooked, paper co-author Peter Olson told IFL Science. "But the report is not that there is a new health risk that people should be worried about," he said.