Medicine: Stop That Germ!

Rapid-fire discoveries are revealing how the body's immune system endlessly fights off disease -- and occasionally goes awry

It's a jungle out there, teeming with hordes of unseen enemies. Bacteria, viruses, fungi and parasites fill the air. They cluster on every surface, from the restaurant table to the living-room sofa. They abound in lakes and in pools, flourish in the soil and disport themselves among the flora and fauna. This menagerie of microscopic organisms, most of them potentially harmful or even lethal, has a favorite target: the human body. In fact, the tantalizing human prey is a walking repository of just the kind of stuff the tiny predators need to survive, thrive and reproduce.

Humans are under constant siege...

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