Is Cancer Contagious? Well... it depends...
Fortunately, cancer has NOT been found to be contagious in humans... yet...
At this time, the only cancers I am aware of that is contagious are found in the animal world. And even in that world, there are only 4.
1) Devil facial tumour disease (DFTD) which causes a contagious cancer of the face in Tasmanian Devils ultimately resulting in death. The cancer originates in the Schwann cells and is suspected to spread when bits of cancer comes into contact with an open wound via biting (which Tasmanian Devils do a lot of to each other).
2) Canine transmissible venereal tumour (CTVT) is passed between dogs via sexual activity and has been known to science for about 100 years.
3) Contagious reticulum cell sarcoma of the Syrian hamster which can be transmitted from one Syrian hamster to another by means of the bite of the mosquito Aedes aegypti.
4) Clam leukemia affecting marine bivalves.
SO... the answer to whether cancer is contagious is YES!!! But not (so far) in humans.
Read more about how cancer may become contagious in this Harper's Magazine article published in 2008 as well as this New York Times article in 2016.