It's not very relevant here, but curiously some cancers are, in fact, contagious organisms. The most famous example is the devil facial tumour disease. Luckily cases of transmissible cancer in humans are extremely rare (if you count only transmission of cancer itself and not the cancerogenic agents).

Another curious case of "cancer being an organism" is the HeLa line derived from cervical cancer cells taken from a woman called Henrietta Lacks.