In 2015, a man was found to have 'cancer' growing in him, but the cancer cells were not his. His 'cancer' cells were found to have come from tapeworms living in his body. The tapeworm cells had invaded his tissues and formed tumorous growths. The man was already very sick and weakened due to infection with HIV, and it is thought that his body was not able to fight off the invading worm cells.
Wikimedia Commons https://commons.wikimedia.org/wiki/File:The_diseases_of_infants_and_children_(1919)_(14780207951).jpg