“A transmissible cancer is a cancer cell or cluster of cancer cells that can be transferred between individuals without the involvement of an infectious agent, such as an oncovirus. The evolution of transmissible cancer has occurred naturally in other animal species, but human cancer transmission is rare. This transfer is typically between members of the same species or closely related species.”
There’s a whole plot-thread based on this concept in the Baru Cormorant series by Seth Dickinson, taken to some really weird extremes.
Just added to my ever growing “to read” list!