BY ACCIDENT and by design, human bodies are hard to break into. They are sur­rounded by defen­sive walls (the skin, the lining of the gut, the bar­rier between the blood­stream and the brain), patrolled by sen­tries (the cells and anti­bodies of the immune system) and filled with mazes, such as the intri­cate net­work of blood cap­il­laries that con­nects the arteries to the veins, deliv­ering food and oxygen to cells as it does so.

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