Let’s say, for instance, someone with lung cancer coughs, and a few cancer cells are present in the aerosol, and is breathed in. Or, if someone has a cut, and lives with a family member who has cancer, and cancer cells end up in the cut.
Cancer cells are a bit like HIV in that they evade the immune system by releasing some chemical to trick it. Hence why in an infamous unethical experiment after the war, people injected with another person’s cancer developed cancer. So even if it is another person’s cancer cells, the cancer will metastasize in the body.
This would make cancer the most overlooked infectious pandemic in all of human history.
Just for future reference: A question mark at the end of sentence means you do not know the answer and are requesting information from others.
It’s not supposed to be used as an invitation to argue with everyone who says something you don’t want to hear.