Imagine a friend of a friend brings his family to stay with you — his family of tiny survivalists. For weeks or months you all live quietly side by side with no problems. You share meals. Your kids play together.
Then one day you get sick — maybe felled by a bad cold or the flu. Suddenly certain the end is near, your jittery houseguest breaks out an armory's worth of chemical weapons. He abandons his community to save himself and hunt for a new home, wreaking havoc on the way out the door.
That's essentially the story line microbiologists in Buffalo, N.Y., have now worked out for why colonies of Streptococcus pneumoniae bacteria (aka pneumococcus) can camp out happily and harmlessly for months at a time in the nose and throats of humans, then abruptly turn on their hosts. The germs sometimes trigger painful earaches or even meningitis in kids, and often pneumonia in the elderly and others with weak immune systems.
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