Doctors believe that the man, 41, developed the condition because his immune system was weakened by HIV.
The Colombian man was first seen by doctors in 2013, who found that he had tumours across his live, lungs and elsewhere in his body.
But further investigation revealed that the tumours - some of which were 4cm across - were cancerous cells that were not human.
Dr Atis Muehlenbachs, from the US Centres of Disease Control said that it ‘didn’t make sense’ and that he was left in ‘complete disbelief’ after discovering what the man was suffering with.
Speaking to the BBC, he said: ‘This has been the most unusual case, it caused many sleepless nights.
‘It should have been obvious this was cancer or an infection and not being able to tell between the two for months is unusual.’
When doctors finally did determine what was wrong with the man, he died three days later.
An expert in the tapeworm, called Hymenolepis nana, said: ‘There is something very special about this species.
‘It is able to carry out its whole lifecycle in one host and that is absolutely unique.’
Dr Peter Olson, who works at the Natural History Museum, explained that more than 90% of the worm’s body is designed to reproduce, releasing thousands of eggs each day into the hosts digestive tract.
It is thought that one of these eggs, travelled through the lining of the man’s intestines, and became cancerous after mutating.
Over 75 million people are thought to carry the tapeworm, but it becoming cancerous is extremely rare.