Everyone has mitochondria--the famous "powerhouse" of eukaryotic cells can hardly be considered a disease. But mitochondria may be relicts of an ancient invasion, an epidemic that took place 500 million years ago. Wolbachia, a bacterial group related to the ancestor of mitochondria, infects a wide range of insects and manipulates host reproduction to promote its own survival. A posse, with $5 million from the US National Science Foundation, is on its trail.