It was Darwin who first taught us that evolution meant survival of the fittest. Darwin’s battle has been fought for billions of years and the survivors are intricate and well-honed biological machines, perfectly fit to their respective ecological niches, and armed to the teeth with the physical and chemical weaponry that got them this far. In this tapestry of life, humanity is no different. In every human body is a constant battle repelling the microorganisms and environmental toxins being inhaled with every breath. Our defenses are formidable. A standing guard of defensive combatants numbering in the trillions rapidly attacks any foreign agents that enter the body. Some combatants are designed to flag invaders as foreigners, others to rip the targets apart with tiny molecular scissors. Still others are charged with preserving the sanctum sanctorum of any living organism—the genetic material. These single-minded scribes read and re-read our genetic blueprints to make sure no foreign instructions have penetrated our defenses. As a result, the body is not a hospitable place for foreign biological material. It is this feature that prevents the vast majority of bacterial and viral invaders from wreaking havoc on our bodies and constantly snuffs out potential cancers when they are little more than mistakes in a copy of our genetic instructions. Yet, this same biological xenophobia is now standing in the way of science and medicine.