In 2013, doctors in Colombia came across a puzzling case. A 41-year-old man came in with what appeared to be cancerous tumors in his lungs and lymph nodes. But while the standard biopsies showed that the tumors contained cancer cells, they also found something rather disturbing: the cancer cells were not human.

It took years of research, but doctors with the Centers for Disease Control and Prevention's Infectious Disease Pathology branch finally unlocked the mystery. The cells were definitely cancerous — they were packed in tightly and multiplying quickly. But they were also fusing together, something that human cancer cells just don't do. Finally, they matched the DNA from the cancer cells with those of the parasitic tapeworm, Hymenolepis nana.

This man had essentially caught cancer from a parasite.

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Unfortunately, this patient died before doctors truly understood what was wrong with him. Researchers are still unclear about what type of treatment they would use to treat cancer caused by a tapeworm. Drugs normally used to treat tapeworm might not be effective against cancerous tapeworm cells, and cancer medications normally target human cancer cells. Would they have the same effect on tapeworm cells?

So how does one "catch" cancer from a tapeworm? Researchers don't really know how the disease was transmitted, but they do know that this particular tapeworm, H. nana, infects about 75 million people around the world, making it the most common tapeworm infection in humans.

The parasite is transmitted when people ingest food contaminated with bugs or mouse scat, or when they ingest feces from infected people. Gross, no doubt, but more common than you want to know, especially in areas where hygiene and sanitation may not be the best.

Most people's immune systems can fight off a tapeworm infection, but when a person with an already compromised immune system becomes infected, the parasite thrives.

That 41-year-old man from Colombia? He was HIV-positive.

The theory is that this combination of parasitic infection and weakened immune system allowed the tapeworm to proliferate unchecked to the point where the cells became cancerous and then spread that cancer to their host via tumors.

While researchers agree that this man's illness was rare, they don't think it's an isolated case.

“We think this type of event is rare," said Atis Muehlenbachs, staff pathologist in CDC’s Infectious Diseases Pathology branch. "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.”