It doesn't take much to be a vile, bloodsucking pest. You, human, have three billion base pairs in your genome, but the body louse—which has been a typhus-spreading scourge of humanity for millennia—carries just 108 million. That's what scientists say today in a study in the Proceedings of the National Sciences that describes how they sequenced the body louse genome. Because the body louse (a separate creature from the head or pubic louse) lives entirely on humans, hatching in our clothes and eating our blood, its genome can get away with being so streamlined, study author Barry Pittendrigh says: