A 41-year-old Colombian man was given cancerous tumours by a tapeworm living inside him, doctors have discovered – the first known report of someone becoming ill from cancer passed on by a parasite. Scientific American reports that the man eventually died from complications relating to HIV, and the weakening of his immune system caused by HIV was likely to be a factor in allowing the tapeworm cancer to spread.

At first, the tumours baffled local doctors: the growths exhibited some of the characteristics of cancer cells, but they were 10 times smaller than the cells you would expect to find in a human and packed very closely together. They turned to the US Centres for Disease Control and Prevention (CDC) for help and it was then that the link to a tapeworm was discovered.

"In the initial months, we wondered if this was a weird human cancer or some unusual, bizarre emerging protozoa-amoeba-like infection," the CDC's Atis Muehlenbachs told the Washington Post in an interview. "Discovering these cells had tapeworm DNA was a big surprise – a really big surprise… this is the first time we've seen parasite-derived cancer cells spreading within an individual. This is a very unusual, very unique illness."

Here's what researchers at the CDC think happened: the Colombian man initially ingested some microscopic tapeworm eggs, probably from food that had been contaminated by mouse droppings, insects or human faeces. Those eggs then multiplied rapidly in the gastrointestinal tract because the man's immune system had already been compromised by the HIV infection – the cells then spread to other parts of the body.

What's not clear is whether the cells were already cancerous or whether some kind of biological reaction caused them to develop into tumours. In fact, 'cancer' may not even be the correct word here, because this is so different from what we normally use the term for: Muehlenbachs says "an infection with parasite-derived cancer which causes a cancer-like illness" may be the more appropriate (though much more verbose) term.

"Can you say a worm has cancer? That's a philosophical question how you define this," he adds.

However scientists end up defining this newly discovered condition, it has some important things to teach us about cancerous cells: up until now, it wasn't thought possible for parasites to develop cancer, let alone pass them on to humans. What's more, cancer isn't considered a transmissible disease, though a few such cases among dogs and Tasmanian Devils have previously been recorded.

The death of this Colombian man has put some of those theories into doubt, and scientists are now calling for more to be done to diagnose the disease and collect data about it in developing nations – if there are more cases like this, we should be able to understand how it is being caused. The results of the study have now been published in The New England Journal of Medicine.