Cancer is typically a threat that comes from within. Mutations to our own cells can sometimes cause them to begin to multiply out of control while simultaneously avoiding the body’s natural defense mechanisms. When this happens, the resulting mass is called a tumor, and it’s usually fatal if not treated.
Thankfully, tumors cannot typically spread beyond their hosts, like bacteria or viruses do. Our bodies are primed to reject foreign cells, and this innate immune response protects us from tissues — including tumors — that come from other people or animals.
That’s not always the case in animals, where there are a few kinds of cancer that are readily transmissible — though never to humans. And some viruses can lead to cancer, making them an indirect mechanism of cancer transmission. But human cancer cells themselves don’t typically jump from host to host.
