Genes that help harmful germs thrive in the warmth of the human body apparently arose from DNA that enables microbes to survive in superheated deep-sea vents. Scientists at the Japan Agency for Marine-Earth Science and Technology compared the genomes of two deep-sea bacteria with those of Helicobacter, responsible for ulcers, and Campylobacter, the leading food-borne cause of diarrhea. According to the researchers, genes that likely help deep-sea bacteria maintain symbiotic relationships with other vent-dwelling organisms assist their gut-dwelling relatives in evading immune systems. Enzymes that help vent microbes live off hydrogen enable Helicobacter and Campylobacter to do the same in the digestive system. And like their harmful kin, deep-sea bacteria have few DNA repair genes, allowing frequent mutations to occur and enabling the microbes to adapt quickly to changing conditions or to resist immune responses. The researchers suggest the human-harming bugs evolved from deep-sea ancestors and later acquired more virulence factors while living in symbiosis with animals. The Proceedings of the National Academy of Sciences USA published the findings July 17.