It's hard to cure cancer definitely because cancer cells will always be trying to out-evolve any treatment. Anything that quickly kills 99% of cells, unfortunately, lets them select for survival traits really fast.
A more common transmission mechanism is itself a virus (https://en.wikipedia.org/wiki/Oncovirus) .
But, remember unlike bacteria or a virus, cancer isn't typically "invading" and you aren't "hosting". It's a malfunction of your own cells resulting in unregulated growth. For the most part it isn't really something you "get", so much as a failure mode of you as a system.
We do use the terminology "invasive" but not because it came from outside the body, rather that it is leaving one organ or tissue and entering another.