Bartonella are Gram-negative hemotropic bacteria that infect a wide range of mammals. At least 14 Bartonella species or subspecies have been reported to be pathogenic for humans and animals. Wild and domestic animals represent a large reservoir. Reservoir species usually display chronic bacteremia. This explains some aspects of the epidemiology of these infections, and especially vector-borne transmission. The molecular mechanisms of persistent infection have clinical consequences both for occasional hosts and for human and animal reservoirs. An increasing number of clinical cases are being described in reservoir species that were previously considered to remain asymptomatic.