When two people become a couple, the brain extends its idea of self to include the other; instead of the slender pronoun “I,” a plural self emerges who can borrow some of the other’s assets and strengths. The brain knows who we are. The immune system knows who we’re not, and it stores pieces of invaders as memory aids. Through lovemaking, or when we pass along a flu or a cold sore, we trade bits of identity with loved ones, and in time we become a sort of chimera. We don’t just get under a mate’s skin, we absorb him or her.