Doctors see them as harmless hitchhikers at best and dangerous pathogens at worst. But a new study of mice shows that herpesviruses, which most of us carry for life, may have a surprising benefit: They offer protection from bacterial pathogens, including the one that causes plague. The effect is a rare example of a beneficial relationship between a virus and its host, the researchers say. Eight human herpesviruses are known, and most people are infected with several of them at an early age. They can cause serious disease: The cytomegalovirus (CMV), for instance, can blind people with compromised immune systems,...