Most cancers—the vast, vast majority—cannot be transferred from one person (or animal) to another. Unlike viruses or bacteria, which are foreign agents designed to infiltrate our bodies, cancer arises when own cells start to grow out of control. This makes it difficult for our immune systems to recognize a tumor as a threat, because all the cells inside are technically part of us. Our immune system is only supposed to attack foreign bodies (though it often screws this up in the form of allergies and autoimmune diseases), so it sees cancer as harmless cell growth instead of a dangerous infiltration. All human cells carry a molecule on their surface called the major histocompatibility complex, or MHC, whose job is basically to tell your immune system “hey, I’m a part of you, don’t kill me.” This is how your body avoids killing itself.