In the first case of its kind, a man has died from cancer not caused by his own cells, but from those of a parasite.
In 2013, a 41-year-old Colombian man went to a hospital with trouble breathing and a fever. He'd also stopped taking his HIV medication months before, which weakened his immune system.
SEE ALSO: South African student steals bus to get to final year exam
Upon examining him, doctors discovered his lungs were filled with tumors. A Centers for Disease Control and Prevention pathologist told NPR, "It looked like cancer, but the tumors were composed of cells that were not human."
"We knew invertebrates can grow tumors, but the fact that one can invade and disseminate in a human and make them sick just really, really defied belief," says the pathologist.
The mystery cancer cells were 10 times smaller than human cancer cells.
Researchers found the man had tapeworm DNA in his system, and further tests confirmed the cancer started in the tapeworm.
SEE ALSO: Man with 'worst headache' of his life discovers tapeworm living in brain
Unfortunately, the man died just days after the discovery, before he potentially could have been treated.
The researchers published their findings in the New England Journal of Medicine Wednesday with the hope that it will help treat more cases.
While the finding is rare, the researchers noted HIV and tapeworm infections are widespread in underdeveloped countries, and more cases may be going undiagnosed.
RELATED -- See foods suspected of causing cancer: