A new way to control a powerful but finicky process called the polymerase chain reaction raises the possibility of a “DNA photocopier” small enough to hold in your hand. Such a device could identify the bacteria or virus causing an infection even before the symptoms appear.

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A new way to control a powerful but finicky process called the polymerase chain reaction raises the possibility of a “DNA photocopier” small enough to hold in your hand. Such a device could identify the bacteria or virus causing an infection even before the symptoms appear.