How exactly is cancer adaptive anyway? Think about how natural selection works: genes that are better at surviving and propagating themselves survive and propagate themselves. Bacteria evolve because they're an independent genome that infests other organisms but cancer is just a failure mode of biology.
Cancer is adaptative in the same way. Tumors are populations of cells with variation (forget the part where your teachers said tumors are composed of clones, that's garbage). When you apply a selection, a small number of the cancer cells survive. Usually there are several different subpopulations. Then they grow, you end up with tumors that are mixed populations of different subtypes. Then the cells start growing again.
Saying cancer is just a failure mode of biology isn't given cancer enough credit. It's more like cancer is a side effect of complex organisms, which is nearly impossible to eliminate, without also eliminating a number of highly advantageous features of cells.
But they're not transmitted from host to host so the only way it would be passed from patient to patient is if a patient himself had genes for resistant cancer and that somehow made him more likely to spread his genome.
