Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Coat of a cheetah. Cheetahs and other big cats are often chronically infected with Helicobacter acinonychis, a "sibling species" of the human gastric pathogen H. pylori. Representative genes from H. acinonychis from captive big cats in the United States and Europe have been sequenced, and strains able to chronically infect mice and coexist with certain H. pylori strains in vivo (potentially exchanging genes with them) have been identified. Comparisons of closely related bacterial species may provide insights into evolutionary mechanisms and histories (here, including H. pylori's origin as a human pathogen) and help identify determinants in pathogens that are important in infection and disease (See related article on page 356).