Members of the genus Bartonella are maintained in many domesticated and wild animal hosts. Bartonella henselae, the best understood species, infects housecats and other members of the Felidae. Additional species of Bartonella are found in cats, dogs, livestock, rodents, rabbits and other wild and domesticated animals. In immunocompetent humans, B. henselae causes cat scratch disease, which is most often a relatively benign and self-limiting illness. In contrast, B. henselae infections are often severe in immunocompromised individuals, and can be fatal without antibiotic treatment. Other species of Bartonella have also been linked occasionally to human illnesses, with varying levels of evidence for a causative role. The significance of Bartonella spp. as pathogens for animals is currently unclear. The vast majority of infections are asymptomatic, and although these organisms have been implicated occasionally in illnesses, proving a causative role is difficult.