Medicine: The Ultimate Parasite

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The millions of invaders are borne in by air or water. Judged by the ordinary signs of life—growth, motion, the need for food—they are dead. One by one, they find individual targets, which may be a thousand times bigger than their sub-microscopic selves. In such an unequal contest, they should have little chance. But these invaders are superlative saboteurs; they are virus particles, and their targets are cells in the human body.

Alongside the cell it is about to attack, the invader acts like an enemy agent skulking outside a big factory....